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Determining the gestational age and date of birth by ultrasound - how accurate will the calculations be? due date calculator

The birth of a child is a long-awaited event for every woman. Barely pregnant, the expectant mother is trying to figure out when her baby will see the light. There are many methods for determining the date of birth. You can calculate this important day in the life of every married couple using the Negele formula. In addition, you can determine when the baby is born, based on data on ovulation or conception. The calculation of the expected term of delivery in gynecology is carried out according to the indications of ultrasound, the first movement of the fetus or the weight gain of the future woman in labor.

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Methods for determining the PDR (preliminary date of birth)

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Often, the expected date of birth (hereinafter - PDR), set by the woman herself, differs from the day the baby is born, which is calculated by the doctor. This situation is explained by the fact that there is an obstetric and embryonic (in other words, true) gestational age, calculated on the basis of information about conception. The embryonic period is usually shorter than the obstetric period by about 14 days. In the latter case, we are talking about the date, the determination of which the doctor carries out, guided by the first day of the menstruation preceding the conception of the child.

In obstetric practice, this concept is used. Establishing the size of the fetus, the time of the necessary examinations, the day of the start of maternity leave, as well as delivery, is carried out exclusively on the basis of the obstetric gestational age.
The effectiveness of a screening study, a diagnostic method that allows timely detection of congenital abnormalities in a child in the womb, depends on how accurately the gynecologist determines when the patient will begin labor activity.

When a future woman in labor first visits the gynecological office, the doctor calculates the expected date of delivery. This procedure is based on the application of several techniques.

By date of last menstrual period

They will find out after how much time the long-awaited baby will be born, according to the date of the menstruation preceding the conception (regulatives). To do this, from it in reverse order, you should count the 3-month period and add 7 days. The resulting number is the preliminary due date. Most doctors, when using this technique, resort to the help of special obstetric pregnancy calendars. It is difficult to accurately calculate the date of birth by menstruation in situations where the future woman in labor has an unstable menstrual cycle or when it is regular, but too long.

For example, if a patient has a 35-day interval between the previous and subsequent bleeding, and not 26-28 days, as in most cases, then most likely she will be able to conceive a baby not on the 14th day of the menstrual cycle, as happens in standard situations, but only on the 21st day. With this in mind, when calculating the estimated term of delivery by this method, it will be revealed that the woman in this situation will have time to give birth 7 days earlier than the true estimated date of birth of the baby.

Ovulation date or conception date

During pregnancy, it is quite easy to predict the date of birth, if you have reliable information about the moment at which the egg was fertilized. To determine how much time is left before delivery, you need to add 266 days to this number - that is exactly how much time is needed on average for the full development of the fetus. Despite its apparent simplicity, this method of determining the due date has several subtleties.

If a pregnant woman knows exactly the date of ovulation or sexual intercourse during which she became pregnant, it is still impossible to speak with complete certainty about the day of conception. The average lifespan of spermatozoa is up to 7 days, the ability of a mature egg to fertilize remains for 2 days after ovulation.

As a result, even taking into account the exact date of sexual intercourse or the release of a mature egg from the ovarian follicle, it is impossible to reliably calculate at what specific moment conception occurred, since this could happen later. In this regard, doctors consider this algorithm for determining future births approximate and use it to calculate the approximate moment of delivery.

According to ultrasound

This technique allows you to most correctly set the date of birth. For the first time, ultrasound is performed at 4-5 weeks of gestation. In the uterine cavity, using ultrasound, a fetal egg is determined, which is a small black circle, in which subsequently, at 6-7 weeks, an embryo appears. It looks like a small strip in which you can detect a heartbeat.

The most accurate is the PDR, determined on the basis of the coccyx-parietal size, which is the size of the embryo, that is, the maximum distance from the highest point of the crown of the fetus to the bottom of the coccyx. After the 12th week of pregnancy, in the process of ultrasound, the calculation of the due date is carried out taking into account the parameters of the baby's head and tummy, as well as the length of his arms and legs.

Up to 9-10 weeks, the fetus develops proportionally, so its size in women with the same gestational age is usually the same. In the future, this indicator begins to vary in future women in labor.

The difference in values ​​in this case depends on many factors:

  • the nationality of the woman;
  • her weight;
  • body weight of both parents at birth.

In other words, the less time remains before childbirth, the more noticeable are the differences in the intrauterine development of a fetus that does not have defects. In some cases, fluctuations in the parameters of children of the same age in the mother's womb can range from 14 days or more. This discrepancy makes it impossible to reliably establish the moment of birth according to the results of ultrasound.

By the first movement of the fetus

You can calculate the preliminary date of delivery using information about the first movement of the fetus. For the first time, a woman who is preparing to become a mother for the first time begins to feel signs of a baby in her womb at about the 20th week of pregnancy, and a future more experienced woman in labor at the 18th week.

Doctors recommend that patients remember this important point, information about which must be entered in the exchange card. To establish the PDR, 20 and 22 weeks are added to the indicated number: for patients who are pregnant for the first time and for women who have already given birth, respectively.

Despite its simplicity, this method is rarely used in modern gynecology. The unreliability of its results is explained by the fact that future women in labor at different intervals for the first time feel the movements of the fetus.

Many factors influence this:

  • sensitivity of the expectant mother;
  • features of her physique;
  • Lifestyle;
  • the presence or absence of motherhood experience.

Some future women in labor feel the first tremors in the abdomen as early as 15-16 weeks, others only after the 20-week period. According to statistics, this happens much earlier in a slender woman than in an overweight expectant mother. Those pregnant women who lead a very mobile lifestyle and devote a lot of time to work often feel the movements of the fetus for the first time later than those who are much less busy and live a measured life. Primiparous women often take the first movements for indigestion, and more experienced expectant mothers immediately determine if the baby in the womb first made itself felt.

By weight gain

If before the introduction of the embryo into the uterine cavity, it was deep frozen with liquid nitrogen (cryopreservation), then to calculate the embryonic gestational age, 5 days should be added to the date of the final stage of IVF - this is how much time the embryo needs to develop before freezing. To establish the obstetric period in this situation, 14 days are counted from the true forward and only after that the date of delivery is determined.

What deviations from the deadline are considered normal?

All of the above methods of establishing the date of delivery are conditional. None of them will be able to accurately answer the question on what specific day the long-awaited baby will be born. According to statistics, only in 17% of cases this event occurs at a predetermined moment.

When a woman finds out about her "interesting" situation, then naturally she does not leave the thought of how the pregnancy proceeds in general, this also applies to the birth of a baby. In addition, one cannot but add here another important and significant event for every woman - when the baby is born, which is no less exciting event. The abbreviation PDR used means nothing more than the expected date of birth.

Of course, one can talk endlessly about this, for example, someone begins to guess in order to mentally prepare for the upcoming birth, and someone does not take any measures at all, referring to nature, namely, when it happens, then it will happen.

PDR - what is it? Peculiarities

Of course, many women believe that it is not difficult to find out the date of birth. After all, all that needs to be done for this is to add the period of pregnancy to the date of conception and you can get the desired date. But not everything is as simple as it might seem.

First of all, it should be noted that not all women know the exact date when the contact actually took place, so you should not forget about it. In general, according to experts, it is almost impossible to calculate even the approximate date of birth, doing this by the date of conception.

Even if you know exactly the date when the conception took place, but you cannot accurately add this or that period to it, this is due to the fact that the body of each woman is individual, which should not be forgotten. Moreover, the same applies to the future baby.

Attention: Never forget, especially if the pregnancy is the first, that in reality there are many different factors that can delay the pregnancy, and in some cases, everything happens differently, they allow you to shorten it.

EDD by the date of the first day of the last menstruation

If we consider this method as a whole, then it should be noted here that it is not only the most relevant and frequently used. If you want to calculate the expected date of delivery (ED) during pregnancy, thereby using this method, then know that this can be done in various ways, for example, using the Nagel formula.

Here you need to take into account the start date of the last menstruation, then subtract three months, add seven days to the result that you get. Moreover, there is another equally common option, when 280 days are added to the day of the onset of menstruation, namely forty weeks.

Here you have to decide for yourself which option suits you best. Of course, as you yourself must understand, this method of calculation will never suit those women who are constantly faced with an irregular cycle.

Ultrasound result

Of course, if all sorts of fortune-telling, assumptions do not suit you, then you can always choose the most effective method that allows you to get answers to your questions. As a rule, a specialist conducts an ultrasound examination, as a result of which, having studied the size of the fetus, he will be able to guess when the long-awaited miracle will actually happen.

As for late pregnancy, the size of the fetus will depend both on the duration of pregnancy and, accordingly, on the individual characteristics of the baby.

In addition to all of the above, the expectant mother should understand that registering almost from the very beginning of pregnancy, you can not only get answers to your questions in terms of calculating the PDR, but also identify various deviations.

As for statistics, as a rule, about five percent of women usually give birth at the appointed time, all the rest more and more often deviate from the cherished date.

Calculation of the date of birth

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Calculation of the date of birth

Pregnancy is the most desired and long-awaited event for any woman. Therefore, it is not surprising that long before documentary confirmation (two treasured stripes on the test) of an “interesting” situation, each of us already feels changes in the body.

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Select the date of conception for online determination of the date of birth

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After your gynecologist makes a pregnancy diagnosis and determines the exact date of conception, it will take only 280 days or 10 obstetric months until the fetus fully matures. At the same time, it is not necessary to strictly focus on these indicators. They are very individual and depend on the physiological characteristics of a particular woman and on the environment in which she bore a child. It is known from practice that only 4% of babies are born clearly after 280 days. The rest, due to various factors, may be born earlier or later than the term. In order to find out the approximate date of birth of your baby and determine the so-called gestational age, they are guided by the first day of the last menstruation.

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Do you have more questions? Read their answers below:

1. How accurate is the calculation for the day of conception

To understand the accuracy of the calculation, you need to decide on the calculation method. It's no secret that conception is possible only during the period of ovulation, during which a mature egg is released from the ovary. The duration of the menstrual cycle is an individual feature of each woman. On average, it can be from 28 to 35 days.

Ovulation occurs in the middle of the menstrual cycle. The exact date can be determined by ultrasound, using special home tests for ovulation, or by measuring basal temperature. Some women on this day have special signs by which she can determine that today is ovulation. It is on this day that conception occurs, which may not coincide with the day of physical intimacy, since spermatozoa are able to maintain their viability in a woman's body for a week.

Knowing the day of conception, which is also a tribute to ovulation, it is quite easy to calculate the date of birth: you just need to add 266 days. This will be the expected date of birth. The accuracy of these calculations will directly depend on the accuracy of determining the date of ovulation. If your cycle is not constant, and you are not sure about the date of conception, then this technique may give some error.

2. How to calculate the date of birth by menstruation

This method of calculating the expected date of birth is used in antenatal clinics. It is also called the obstetric term. To determine the expected date of birth, you need to know the date of the beginning of the last menstruation. This day is the starting point for the calculation. If the duration of the menstrual cycle is 28 days, then you can use the Negele formula, according to which the date of birth is calculated as follows: three months are subtracted from the day the last menstruation began and seven days are added. You can also use another technique: 280 days are added to the date on which the first day of the last menstruation fell. According to scientific data, it is this period that is necessary for the maturation of the fetus.

But if your cycle is longer or irregular, then calculating the date of birth by menstruation will not be entirely correct. If the duration of the menstrual cycle exceeds 28 days, then childbirth is likely to occur later, if vice versa, then earlier. If the cycle is not regular, then this method of determining the expected date of birth will be rather indicative and cannot be used as a basic technique.

3. How to calculate the date of birth by the first movement

Often, to determine the expected date of birth, such a landmark is used as the very first movement of the baby inside the abdomen, or rather the moment when the mother was able to feel the movement of her baby. Therefore, it is also called intuitive and experts consider it inaccurate. Moreover, if a woman gives birth for the first time, then she will most likely feel the first stirring at the twentieth week of pregnancy. If a woman is about to give birth again, and she already knows what exactly she should feel, then the first movement of the fetus in most cases is felt at the eighteenth week. Now, in order to calculate the date of birth by the first movement, if the pregnancy is the first, it is necessary to count exactly 20 weeks from the date of the first movement. If repeated births are coming, then only 18 weeks should be counted.

As practice shows, quite often many primiparas and women who give birth again begin to feel the movement of their baby already for a period of 15-16 weeks. But there are also mothers who felt the first movement of their baby only for a period of 21-22 weeks.

Do not rush to apply this technique in practice to determine the expected date of birth, since in fact it is not known which category of future mothers you belong to.

4. How to calculate the date of birth by ultrasound

The most accurate method for determining the date of birth is considered to be diagnostics by ultrasound (ultrasound), but it must be used at a very early date, no later than the twelfth week. This method allows diagnosing the gestational age with an accuracy of one day, thus indicating the approximate date of conception and, accordingly, the date of birth.

Later, the gestational age is determined by the size of the head and limbs of the baby. But this conclusion does not allow accurately diagnosing the expected date of birth, since each child develops absolutely individually, having a different weight not only at the time of birth, but also during its intrauterine development. Therefore, when determining the date of birth in the second trimester, an error of up to seven days appears, and ultrasound in the third semester makes it difficult to judge the gestational age, based on the intrauterine dimensions of the baby.

Thus, in order to accurately calculate the date of birth by ultrasound, it is necessary to undergo an examination during the first three months of pregnancy. Otherwise, the intrauterine dimensions of the child will not allow you to accurately determine the date of the expected birth.

EDD is an abbreviation used by doctors, which means the following: Estimated Date of Delivery. Yes, it is conjectural, because even with the help of all the cutting-edge technologies, the date of birth can be calculated with a small error. For example, if a woman had problems with hormones before pregnancy, then her day could be shifted, and the pregnancy did not come when it was supposed to. Another reason for the error may be - usually childbirth with it always occurs earlier than expected. It turns out that the PDR is an indirect date, from which it is worth starting, but do not forget that childbirth can occur either a little earlier or a little later, which will be the absolute norm.

Doctors define several types of definition of PDR:

  • on the day of ovulation;
  • according to the Negele formula and the date of the last menstruation;
  • with the help of ultrasound;
  • according to the results of the inspection;
  • by the first movements;
  • according to the height of the fundus of the uterus;
  • by waist size;
  • by date of conception.

Calculation of the EDD by the day of ovulation

A common and erroneous belief is that pregnancy lasts 9 months. In fact, its duration should be calculated in lunar months, and the period will be 10 lunar months. In days, this period will be 280 days.

If the pregnancy is planned, you almost certainly know the date of the last ovulation and, accordingly, the date of conception. To this day, you need to add 280 days, which will be the time when the long-awaited child should be born.

If you do not know when ovulation was, but the cycle was stable, then you need to take exactly the middle of the last cycle and add the same 280 days to this day. But here a small error is already possible, which should be counted on.

Negele formula

This method is considered the most common among gynecologists-obstetricians. This is probably the simplest and at the same time the most truthful way to calculate the ERP. First of all, the doctor asks when was the last menstruation. 3 months are subtracted from the first day of the last cycle and 7 days are added to it.

  • you need to count 3 months, we get July 4;
  • to this date add 7 - PDR on July 11.

But Naegele's formula is effective for women who have a cycle of 28 days - the most common duration, according to statistics. But what if the cycle is from 21 days to 35 days, which is also the norm? It turns out that if, with such a cycle, the date of birth is calculated using the Negele formula, then the error will immediately be plus or minus one week. In such cases, doctors resort to methods that MirSovetov offers to familiarize yourself with below.

PDR on ultrasound examination

It should be noted right away that determining the exact gestational age and setting a more or less correct PDR is possible only if the examination is completed up to 12 weeks. The reason for this is that all people are different, starting from the womb. Some babies are born larger, others smaller, and everything is considered the norm. Only when the child's weight is above average, is there a high probability that the doctor will set a period longer than the present. For children whose weight is somewhat less (the cause of underweight can be both intrauterine growth retardation and the physique of the child), the doctor may mistakenly set the period somewhat less.

To indicate the correct period, it is important to undergo at least one ultrasound before 12 weeks. On examination, the doctor can determine the weeks to the nearest day. Later, with other examinations and examinations, it will be possible to build on the indicators of the first ultrasound.

PDR based on the results of the inspection

The system here is the same as with an ultrasound examination: the doctor can determine the gestational age at the examination only up to 12 weeks. Later, the development of the fetus may be different and there is a high probability of erroneous setting of the term. Already a week after the start of the delay, the doctor will be able to determine the pregnancy and its duration by the shape and size of the uterus. Do not forget that doctors prefer to conduct gynecological examinations on the armchair as rarely as possible during pregnancy.

PDR on the first movements

The child begins to actively move in the womb even at a very early date, but the woman does not feel this, because the fetus is still too small. Standard doctors believe that during the first pregnancy, a woman feels the first movements at 20 weeks, and in subsequent pregnancies at 18 weeks. From this period, you can build on and calculate the estimated date of birth as follows:

  • for primiparas, add 20 weeks to the date of the first movements;
  • during the second and subsequent pregnancies - 22 weeks.

However, often women, especially those who are not pregnant for the first time, tell doctors that they feel the baby's movements at an earlier date, from 14 weeks. Doctors associate these sensations with the activity of the intestines, and setting the date of birth by the first movement becomes difficult.

PDR for the height of the fundus of the uterus

The doctor can accurately set the date of birth during the examination by determining the height of the fundus of the uterus. Throughout pregnancy, the uterus rises higher, at a period of 16 weeks it is located between the pubis and the navel, at 24 weeks in the navel area, at 28 weeks it is slightly above the navel. This examination is not carried out on a gynecological chair, therefore it is safe for mom and baby.

EDD by waist

Very high accuracy from this method can hardly be achieved, because the measurement of the abdominal circumference is very approximate. It depends on the physique of the mother, her initial weight. If the mother is rather chubby, then her volumes will be greater than that of a skinny mother. Also during pregnancy, the appearance of strong ones even in thin women is possible, which provokes high rates in volumes. So in such an inspection it is necessary to consider all objective circumstances.

PDR by date of conception

Also a rather controversial method, because the possibility of making a mistake on a specific date is very high. As you know, a woman can become pregnant only on certain days of the cycle, approximately 12-14 days. Even if the sexual intercourse was single, suppose, on the 11th day of the cycle, then the conception itself could well have occurred on the 12th day. This is due to the fact that sperm cells can live in a woman’s body for up to several days and “wait” for the egg to be ready to accept them for fertilization. Therefore, it is quite possible to make a mistake with the date by a day or two.

Even knowing the date of a possible birth, the child knows better than us when he should be born. Therefore, doctors put the estimated date of birth at 40 weeks, but the appearance of a child from 37 to 42 weeks is considered the norm. And here, indeed, little depends on both the doctors and the woman herself.

Many women, especially those who are expecting their first child, do not understand why the doctor gave this or that date of birth, although according to their calculations, it turned out that the baby should be born earlier. You should not worry, the doctor was not mistaken, you too. How can this be? The fact is that there is a true gestational age and obstetric, they differ from each other by two weeks.

With any pregnancy, the estimated due date is always calculated. There are several methods for calculating the approximate date of birth.
  • Embryonic. This is the real term, i.e. That's how long a normal pregnancy lasts. From the date of conception to delivery, it usually takes 266 days or, in terms of weeks, 38 weeks.
  • Obstetric. This period is usually recorded in the patient's card. The doctor cannot know the date of conception of the child. Medicine is an exact science, it cannot be based on the assumptions of a woman. Therefore, obstetricians-gynecologists use their own method. The countdown is not from conception, but from 1 day of menstruation before pregnancy. This period differs from the true one, its duration is 280 days (forty weeks).

Calculation by day of menstruation

The most common way that helps to calculate the expected date of birth quickly is by the last menstruation.

  • Think back to the day you started your last period before pregnancy. Mark this day on your calendar.
  • Now subtract three months (we step backwards on the calendar).
  • Add seven days to the received date.

Let's say your last critical days began on July 15th. We count down three months - it turns out April 15th. Add 7 days to this date. The date of the birth of the baby is April 22.

You can quite accurately calculate the term of childbirth by the date of menstruation, if a woman has a regular cycle and it lasts from 26 to 28 days. If it is slightly longer, for example 35 days, the result that was obtained will be incorrect. In this case, conception should occur on the 21st day of the cycle, and not on the 14th, as in many women, i.e. labor can start up to 7 days later. If the cycle is short, less than 24 days, then the date of delivery also moves by about 7 days, they should begin earlier.

Calculation using a calendar

If you find it difficult to make these calculations or you doubt that everything is correct, use the birth calendar to calculate this day from the date of the last period. The top line is the time of the onset of menstruation, below it is the expected date of birth.

Or you can use the online calculators that are available on many sites. It is enough to enter the date of the beginning of the last menstruation in a special window, after which the system will give the desired answer. Now online pregnancy calendars are very popular, they will not only help you calculate the date of birth, but also tell you what is happening in a woman’s body. So the expectant mother will be able to monitor the development of her baby, find out what changes have already occurred. If you wish, you can even order your own individual calendar, where it will be scheduled in time as the child grows.

Calculation by date of conception

How to correctly calculate the date of birth if menstruation is irregular? There is another way that will help calculate the date of birth by the date of conception. You need to remember on which day the conception of the child occurred. Then add 266 days to this figure. That is how many days a woman usually bears a baby. You can also calculate the date of birth by weeks of pregnancy, because. sometimes it's hard to count the days. On average, after conception, pregnancy lasts 38 weeks. Using the calendar, it is easy to calculate the due date according to the known date of conception.

Are these methods reliable?

If you calculate the date of birth by the last menstrual period, is the baby exactly born on that day? No, according to statistics, only 5% of children are born on the “settlement day”, this is an approximate date, babies are born within a week (plus or minus) of this day (85%), and about 10% of children may be one more late week.

Also, if you calculate the correct date of birth according to the day of conception, there may be errors. Not everyone can name the date of conception of a child, because. this date sometimes does not coincide with the day when there was intimacy. The sperm can remain in the body and fertilize a woman for 4-5 days after intimacy, i.e. a woman could have sex one day and conceive a child later. But, even if the calculation was correct, the birth may be premature due to various diseases, due to severe stress, etc.

Additional ways to determine pregnancy

  • A trip to the gynecologist. As soon as the expectant mother comes to the doctor, the gynecologist examines her and finds out what her gestational age is. Accurate results can be called for a period of 5 to 12 weeks by measuring the size of the uterus. The blood test of a pregnant woman for hCG is also important. This hormone is produced only in the body of a woman who is expecting a child, at different times its concentration in the blood is different. The doctor has tables that indicate how much hCG should be at one time or another.
  • ultrasound. The physician must measure the size of the fetal egg, as well as the embryo. Up to 9-10 weeks, the indicators for all women at a certain period are the same. From them you can find out the duration of pregnancy. Later, this method will not work, because. after 10 weeks, the size of the fetus in different women can vary significantly, depending on the weight of the mother, on nationality and on other factors that affect the weight of her baby.
  • By the first move. It is believed that a mother expecting her first child feels 1 movement at twenty weeks, and the rest of the pregnant women at eighteen weeks. Based on this knowledge, you can calculate the date of birth: for the first time a pregnant woman, add twenty weeks to this date, and for a woman who has already given birth - 22 weeks. But some mothers feel the movement of the baby as early as 15-16 weeks, while others after 20 weeks, so these numbers will not be accurate.
  • There are many ways to calculate due date. You should not hope that the child will be born on this very day, but it is important for a woman to know at least approximate dates in order to complete all her affairs, pack her things and prepare for the birth of the baby. This information is also necessary for the doctor who leads the pregnancy to make sure that it is proceeding normally.

As we can see, it is not difficult to make such calculations; you can calculate the desired number in just a few minutes.