Tracking down the birth date calculation: methods and accuracy

The anticipation of the arrival of a new family member is immeasurable, and determining the due date plays an important role in this exciting process. Modern medicine offers various methods of calculating the expected date of birth to give expectant parents an idea of when they can hold their baby in their arms. In this article, you will learn more about these methods and how they are used.

Calculation based on the menstrual cycle: a basis for estimation

The Naegele rule, named after the German gynecologist Franz Naegele (1778-1851), is a frequently used method for calculating the most likely date of birth for a human pregnancy. This rule is based on the mother's menstrual cycle, as most women do not know the exact date of conception. Doctors use this method as a basis for estimation by adding 280 days, which corresponds to about nine months, from the first day of the last menstruation. If the exact date of conception is known, the due date can also be calculated by adding 266 days from this date. Birth dates based on the last menstrual period only apply to 30% of births, so they are quite unreliable1.

Difference between gestational age and embryonic age:

There are two common methods for determining the progress of a pregnancy. The first method, known as gestational age, is based on the first day of the last menstruation, as described above. 

The second method, embryonic age, is based on actual conception and is mainly used in research and artificial insemination. The embryonic age is based on the day of conception, which is usually around 14 days shorter than the gestational age. However, embryonic age can vary depending on the individual menstrual cycle and is therefore less commonly used in clinical practice. This method requires more precise knowledge of the time of conception and is often used when this is known, for example in cases of assisted reproduction or with precise conception dates.

Both methods, gestational age and embryonic age, are used to follow the course of the pregnancy and estimate the expected date of birth. Depending on the clinical context and the information available, one or the other method may be preferred.

Ultrasound examinations for more precise results

Ultrasound examinations are an extremely valuable method for more accurately determining the due date during pregnancy. This non-invasive technique makes it possible to directly visualize the fetus and measure various aspects of its development. Especially in the first weeks of pregnancy, ultrasound examinations prove to be extremely accurate.

Research has shown that ultrasound examinations performed between the 11th and 14th week of pregnancy provide the most accurate results2. At this stage, the fetus is sufficiently developed to measure certain parts of the body, leading to a reliable determination of gestational age. Measuring the crown-rump length, head circumference and other parameters allows the medical team to accurately track the progress of the pregnancy.

In addition, ultrasound examinations not only provide precise data on the week of pregnancy, but also provide insights into the health of the fetus. This can help to identify potential problems or developmental disorders at an early stage and ensure the necessary medical care. Ultrasound examinations are therefore not only a valuable tool for determining the date of birth, but also serve to comprehensively monitor the health of mother and child during pregnancy.

The role of measuring the uterus

The palpatory method, where the doctor or midwife assesses the size and position of the uterus by palpating the mother's abdomen, can be used during antenatal check-ups. By assessing the position and size of the uterus in relation to the expected week of pregnancy, healthcare professionals can make a rough estimate of gestational age. This can be particularly helpful in the early stages of pregnancy when ultrasound scans may not yet have been performed.

Thanks to Daysy, I know when I ovulated. How can I now calculate my exact due date?

Using fertility trackers such as Daysy not only help you determine your ovulation, but also help you to get a more accurate picture of your individual menstrual cycle. By incorporating this information into the calculation, you can increase the accuracy of determining your due date and get a better idea of what week of pregnancy you are really in. In this case, it is important to share this information with your doctor. Your doctor can then use this information to make a more precise calculation of your due date and give you a more accurate due date. This can help you and your doctor to ensure the best possible care during your pregnancy.


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What are the first signs of pregnancy?

Basal body temperature is a good way of monitoring pregnancy at a very early stage. If your period is absent, your basal body temperature remains at a consistently high level (approx. 37 C°) for more than 18 days, and there are no disturbances due to illness or similar, it may be an early pregnancy (see illustration). You may even be able to observe a second rise in basal body temperature due to the implantation of the fertilized egg in the uterus. Implantation of the egg takes place around 6 to 12 days after ovulation.

Daysy indicates a possible pregnancy with all lights (red, green and yellow) flashing. Daysy indicates a higher probability of pregnancy when all lights (red, green and yellow) are lit continuously. In your temperature graph in the Daysy app, a possible pregnancy is shown in blue.

Sensitive breasts: A common sign of pregnancy is enlarged breasts that are sensitive to the touch. It is not uncommon for the breasts to tighten or even hurt. This is due to the increased blood flow to the breasts, which is why some women's nipples begin darkening very early in the pregnancy. The same can also apply to the labia. These symptoms are found in 70 to 80 percent of all pregnant women and can occur around a week after the absence of menstruation.

Implantation bleeding occurs around 4-6 days after fertilization and can be one of the first signs of pregnancy. As the name suggests, the bleeding is associated with the implantation of the fertilized egg in the lining of the uterus.

Nausea and vomiting: Probably the best-known and most common early sign of pregnancy is morning sickness. If you get up in the morning with a queasy feeling in your stomach, you're probably wondering: Am I pregnant? The nausea is triggered by the pregnancy hormones, which are now being released in large quantities.


The calculated due date is often an estimate and not always accurate. Many pregnant women give birth to their babies weeks before or after this date. Nevertheless, the due date is important for monitoring the pregnancy and the health of mother and child. The combination of various methods, such as calculation based on the date of ovulation and ultrasound examinations, enables more accurate dating and better preparation for the birth. 

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Author : Dr. Niels van de Roemer


1) Mahendru AA, Wilhelm-Benartzi CS, Wilkinson IB, McEniery CM, Johnson S, Lees C (2016) Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing. Arch Gynecol Obstet 294:867–876.

2) Kagan KO, Hoopmann M, Baker A, Huebner M, Abele H, Wright D (2012) Impact of bias in crown-rump length measurement at first-trimester screening for trisomy 21. Ultrasound Obstet Gynecol 40:135–139.