Dating Pregnancy and Determining the Estimated Date of Delivery

Determining the babys due date and the duration of the pregnancy is an important element in the follow-up appoints during pregnancy. Calculating the correct period of pregnancy helps us to properly track the stages of fetal development and growth. From a different point of view, when we know the estimated date of delivery (EDD), we know when the fetus is mature enough to be born. Due to various factors, in  5% of pregnancies we have premature birth, as well as overcarried pregnancy. The medical team can not be aware of whether or not you are in danger of any of these conditions without knowing the exact duration of your pregnancy and the due date. Some pregnancies require paturition before spontaneous birth begins or well before the due date. In order for the medical team to be prepared and, above all, to minimize the risks in the life and health of the fetus, the exact time and stages of pregnancy are extremely important.

Why do we use the term estimated date of delivery (EDD)? This is the date when the pregnancy is exactly 40 weeks, but whether the birth is going to happen at that point is not clear. Only 5% of women give birth on the exact date, and the rest before or after, that’s why it is estimated.

The above, I think, gave you an idea of the importance of accurately determining the duration of your pregnancy.

The oldest but most inaccurate method for determining pregnancy is based on the Naegele’s formula, which adds 7 days to the first day of the last menstrual period (LMP) and subtracts 3 months. This way of dating the pregnancy is very easy and does not require any additional equipment, medical person or software qualification.

As you understand, the calculations are based on LMP. Not all women, however, have regular menstruation, and even if it is very regular, in most cases we do not know when ovulation occurred. Ovulation occurs in the middle of the cycle between the 12th and 16th day of a 28-day cycle. But various factors can affect menstruation as well as ovulation – travelling, tension, eating disorders, medication, health problems, and so on. We know that the ovum has between 24-48 hours of life after ovulation. The sperm has up to 72 hours of life. At what point will the ovum and sperm meet? From this it becomes clear that the determination of EDD by the Naegele’s formula may be possible to vary by few days, even weeks, from the real moment of the term. LMP can only be used in a very advanced pregnancy after 26-28 weeks of pregnancy (WOP) when other methods are less accurate.

The main method of determining EDD is performed by ultrasound between 11-14 weeks of pregnancy, by measuring the length of the fetus from the head to the seat (without the feet), the so-called crown rump length (CRL). This way of dating pregnancy has been proven by a statistically credible study by Professor Nicolaides in London, Britain, in the 1990s. For pregnancy to be dated under this method, special conditions are required: a trained specialist perfoms an ultrasound examination in order to be able to apply all CRL measurement criteria, a high-resolution echograph, software for calculating the parameters.

It is possible to determine the approximate length of pregnancy by the CRL method before 11 weeks of gestation, but the measurement should be repeated within the correct time – between 11 and 14 WOG to be fully accurate.

The most accurate way to date pregnancy and determine the likely due dat is when we know the exact moment of fertilization. The only case in which we know the date, the hour, and even the minute of fertilization is in in-vitro procedures. Especially when transferring a fresh embryo, this is the most accurate method for dating pregnancy. In this case, we count or most often use software to calculate EDD. Very rarely, when transferring a frozen embryo, we can also date through the CRL.

If the pregnancy is spontaneous, i.e. it is not in vitro and the moment between 11-14 week of gestation is missed, we need to use the head circumference as a method of determining EDD. This method is not as accurate as using CRL but is more accurate than the last regular menstrual period. Here the same conditions apply as to CRL – a trained specialist, ultrasound and software. This method can be used up to 26-28 WOP, after which it is inaccurate and unprofitable.

In multifetal pregnancy, dating is done using the above methods, depending on the method of conception and the due date.


Center for Fetal Medicine

Get informed about the world of Fetal Medicine with Dr. Boris Stoilov.

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