Accuracy of Pregnancy Ultrasound to Predict the Due Date

All pregnant women look forward to knowing their due date. The 40-week countdown is a long period of wait, but they look forward to the day they can hold their little angels in their arms. Your gynaecologist would deduce the due date by making a simple calculation based on the last day of the last period. However, another way of predicting the due date is by a pregnancy ultrasound, which has become very helpful for obstetricians nowadays, especially when the menstrual history is unknown.

It is a given that there will be a marginal difference in the prediction made with the help of ultrasound and the one based on the menstrual history. However, ultrasound gives a closer look into the development of the foetus which can also help calculate the expected date of delivery (EDD), especially when the exact date of the last menstrual cycle is not known.  But, how accurate is the prediction by ultrasound? We shall discuss this and some more factors concerning determining EDD in this article. Read on to know more.

Can Your Pregnancy Ultrasound Determine Your Due Date?

Pregnancy ultrasound does give an idea to the expecting mother about the tentative date of delivery. As mentioned earlier, the due date, which is calculated from the last menstrual cycle, often does not match the due date calculated by ultrasound. The accuracy of the EDD by ultrasound depends on several factors such as the current stage of pregnancy, the quality of the machine, and the position of the baby in the mother’s womb. Therefore, it is safe to say that the accuracy of the ultrasound decreases as the pregnancy progresses. Hence ultrasounds cannot be 100 percent correct and reliable.

When Should You Have an Ultrasound Scan to Determine the Due Date?

An ultrasound can help determine how far you’ve reached in your pregnancy, but only to some extent. Gynaecologists can predict it through ultrasound imaging after six weeks of confirming the pregnancy. One should bear in mind that as the pregnancy progresses, there can be a large gap between the expected date of delivery predicted by ultrasound and the actual delivery date.

Why Is an Ultrasound Scan Needed to Determine the Due Date?

The women who want to be sure of their date of delivery, cannot rely entirely on the manual cervical check (a manual checking done by the doctor to feel the softness and length of the cervix with the finger) to determine the date of the delivery. Pregnant ladies and doctors find it more suitable to conduct ultrasound scans to assess the cervix and the passageway which opens up and reveals if labour is possible when she is nearing the due date. The changes that start happening in the cervix are internal and not external. Therefore, transvaginal ultrasounds can help reveal the shape of the cervix and indicate whether the process of confinement has started. Also, estimating the due date more closely helps the doctor take medical decisions for the health of the mother and the baby. 

How Accurate Do Ultrasound Scans Predict the Due Dates?

Pregnant ladies, these days get frequent ultrasound scans done as it is a safe way to know the measurement of the cervix (the passage that makes way for the baby to be delivered). The scans also help the gynaecologist deduce whether the patient will be in labour soon. Plus, ultrasound is painless and risk-free, and seeing the baby on the screen is also an enjoyable experience. Though all gynaecologists give a due date, it is just a tentative date. Very few women deliver their babies on the expected due date. Hence, the accuracy of the due date by ultrasound is not always absolutely correct either. The estimated due dates calculated by pregnancy scans are sometimes found incorrect even though the technicians are experts in their field.

Can Your Due Date Change During Pregnancy?

Early ultrasound has a negligible margin for error while determining the due date. Generally, the gynaecologists have no way to know the exact time of conception. Therefore, pregnancy is calculated from the very first date of the last menstrual period. The doctor may change the due date based on the result of the first ultrasound, which is fairly reliable. In most cases, the date differing by more than a week mostly occurs in the first trimester. If consecutive ultrasounds show consistent progress of the foetus with no indication of any problems, then there is no reason to worry if, for some reason, the doctor changes the due date.

Do Scans in Early Pregnancy Predict the Due Date More Accurately Than the Scans in Later Pregnancy?

It has commonly been observed that throughout the pregnancy, each ultrasound shows a different due date. Early pregnancy scans are more accurate than the latter ones for predicting the due date. Therefore, doctors refer to the dates and measurements predicted on the first ultrasound. If an ultrasound is done between 18 and 28 weeks of gestation, the margin of error can increase to one or two weeks (minus or plus). As the baby keeps getting bigger in size, it becomes harder to predict the expected due date accurately. So, in the later stages of pregnancy, ultrasound measurements are not conducive. A better estimate of due dates can ensure a safer delivery.

Which Due Date Is More Accurate – LMP (Last Menstrual Period) or Ultrasound Scan?

Ultrasound scans are done for multiple reasons. They are used to document and reassure appropriate and adequate growth of the foetus. The ultrasound report includes measurements of the foetus’ abdomen, head, thighs, and information about the amniotic fluid. Details about the embryonic development as shown by ultrasound scans are more accurate as you can see each stage of the pregnancy.

Predicting the due date on LMP depends on how regular your menstrual cycle is. LMP may differ when the body is under stress, or affected by minor or major illnesses. Also, the method adopted by “pregnancy wheels” to calculate the estimated due date based on the LMP is done by adding 280 days to the first day of the last menstrual period. If the date is not known, using LMP to establish the date can overestimate the duration of pregnancy.

Thus, the due date predicted with the help of ultrasound is different from the one based on LMP. The ultrasound assessment is comparatively accurate in most cases, especially when the scans are done between six to ten weeks. Decisions which are based on third-trimester sonography could be problematic. The gynaecologist would then require closer monitoring to deduce the expected date of delivery, which often, is of no use as the date seldom matches.

Ultrasound, also called a sonogram, is a prenatal test, used by most healthcare providers to check the foetal health and development. A healthy pregnancy is generally 40 weeks from the date of the last menstrual period, but it is observed that babies are delivered between 37 to 42 weeks of gestation. Actually, there isn’t a reliable method to tell exactly when a baby will be delivered. The calculation of the date of conception is just an estimated date. The world of pregnancy and childbirth has revolutionized with many scientific advances in the field. In assisted reproductive procedures, the calculations can be done accurately, but that is not the case for natural conception. An optimal way to determine the gestational age is through ultrasound scans during the first trimester. Thus ultrasound can be called a key tool for determining the estimated due date as well as for confirming the health of the foetus.

Reference:

• WebMD
• Healthline

Also Read: Ultrasound Scan: Is It Safe for You and Your Baby?