How your pregnancy due date is calculated: Methods and the role of cycle tracking.

Pregnancy is a time filled with magic and anticipation, bringing about wonderful changes in your life. The arrival of a new family member brings immense joy, and your due date marks one of the first significant milestones, making your baby feel truly tangible. 

Determining your due date plays a crucial role during this exciting period. It provides a guideline for your personal preparation and serves as an important reference for medical professionals to monitor the development of your pregnancy.

In this article, you will learn more about calculating your due date and how cycle tracking can make a valuable contribution.

What is the typical calculation method for your estimated due date?

For over 200 years, Naegele's rule has been the standard method for calculating your estimated due date and is likely something you will encounter early on in your pregnancy. 

Since many women do not know the exact date of conception, this calculation relies on the mother's menstrual cycle and the date of your last period.

It involves adding 280 days to the first day of your last period. If you know your conception date, your due date can also be set 266 days after that day.

EDD = 1st day of your last period + 7 days - 3 months + 1 year

This period, starting from the first day of the last menstrual period, is referred to as the gestational age. During your pregnancy, you might come across this date abbreviated as 'LMP' (last menstrual period).

The original formula for calculation is based on the assumption that pregnancy lasts an average of 280 days and that the female cycle has a regular rhythm of 28 days. 

In the extended Naegele’s rule, cycle length deviating from the norm is included in the calculation. If the cycle is extended or shortened by x days, x days must be added or subtracted from the calculated date.

EDD = 1st day of your last period + 7 days - 3 months +/- deviation days + 1 year

Naegele's rule relies on a standard calculation that applies equally to every woman. Given the uniqueness of us women, our individual cycle, and our personal pregnancy, it is hardly surprising that only 3-4% of all children are born exactly on the estimated due date set by Naegele's rule (2,3).

Why does this calculation method not work for many women?

The calculated due date, based on your last period, is often inaccurate and relies on assumptions that do not align with current research. 

The calculation does not account for the individuality of us women and our cycles:

Different Cycle Lengths: Only about 13% of women have a menstrual cycle that is exactly 28 days. Most have cycle lengths ranging from 23 to 35 days (4).

Ovulation Timing: Variations in cycle length mainly result from differences in the length of the follicular phase (the time between the period and ovulation) (5). Thus, the timing of ovulation varies significantly from woman to woman.

As you can see, the chances of your cycle being exactly 28 days long with ovulation on the 14th day are extremely low. Additionally, various factors such as hormonal contraceptive use, age-related changes, and BMI can significantly influence your individual cycle.

It is evident that the standardized assumptions of Naegele's rule, which presumes ovulation on the 14th day, does not align with the diversity of the female body and therefore does not reliably predict the exact pregnancy month and due date.

Why is your due date changed based on an ultrasound?

Ultrasound scans enable a more precise determination of the due date compared to calculations based solely on the menstrual period.

When performed between the 11th and 14th week of pregnancy, ultrasounds provide more accurate results by measuring various aspects of fetal development (1)

For instance, crown-rump length or head circumference are used for assessment, leading to a more reliable determination of gestational age.

Your doctor might adjust your original due date based on these results. 

Since ultrasound growth curves are based on average values and fetal development occurs in spurts rather than linearly, this method is also not entirely reliable for determining the exact gestational age (6).

Ultrasound scans provide not only information about the gestational week and month, but also insights into your fetus's health, allowing medical professionals to ensure everything is going well with you and your little miracle.

How can cycle tracking help determine your due date?

Cycle tracking, with the help of the basal body temperature helps you to understand your body's rhythm better and provides valuable information for calculating your due date. 

Modern cycle computers like Daysy or Lady-Comp offer precise knowledge about the timing of your ovulation and the length and regularity of your cycle, allowing you to determine when you conceived more accurately.

With Daysy, you can make a more accurate, individualized calculation of your due date that aligns with your personal menstrual cycle, giving you a much clearer idea of when you might hold your baby in your arms. 

If you have already tracked your  with a cycle computer like Daysy and have information about the length of your menstrual cycles, you have a reliable starting point to calculate your due date. 

How can you calculate your due date using the temperature curve?

According to current knowledge, a pregnancy typically lasts a median of 38 weeks or 266 days from fertilization to birth. By tracking your cycle, you can easily identify your ovulation through the temperature curve in the DaysyDay Cycle App. 

Daysy also reliably indicates a pregnancy at a very early stage. A potential pregnancy is indicated when all the lights (red, green, and yellow) are flashing.

If you have a look at the temperature curve on the DaysyDay App, you can calculate your due date as follows:

EDD = Day of the first higher measurement (after ovulation) + 267 days (38 weeks)

Lady-Comp, our standalone device, takes it a step further. It indicates a possible pregnancy 15 days after ovulation and automatically calculates your due date based on your personal cycle data.

While your body doesn't operate like clockwork and the duration of pregnancy varies individually, insights from cycle tracking offer a significantly more accurate calculation than applying Naegele's rule, especially due to the variable timing of ovulation.

When you visit your gynecologist or midwife, records from your cycle tracking provide valuable information about your health and pregnancy. 

Knowing your body well through cycle tracking lays the best foundation for your pregnancy until you can finally hold your little miracle in your arms.

Your path to motherhood: Achieving a more accurate due date with cycle tracking

The calculation of the due date marks a significant milestone in your pregnancy, often determined using the traditional Naegele's rule, overlooking the uniqueness of each women and her menstrual cycle

This is where cycle tracking becomes immensely valuable. With modern technological devices like Daysy or Lady-Comp, you can gain a deeper understanding of your body and accurately monitor your cycle.

Cycle tracking undoubtedly represents a significant advancement. It allows for a precise determination of your due date by taking into account the uniqueness of your cycle and enables calculations based on the day of your ovulation. 

By accurately identifying ovulation, you can pinpoint your due date accurately. 

This opens the opportunity to cater better to your individual needs and ensures personalized care throughout pregnancy.

At Daysy, we wish you all the best on your journey to motherhood and are ready to support you with our technology and expertise at any time!


(1) Kagan, K. O., 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 in Obstetrics & Gynecology, 40(2), 135–139. 

(2) Komlew, Andrea. (2017). Ist die Naegelsche Regel nach 200 Jahren noch aktuell?. Die Hebamme. 29. 340-344. 10.1055/s-0042-115580. 

(3) Jukic, A., Baird, D., Weinberg, C., McConnaughey & Wilcox, A. (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction, 28(10), 2848–2855. 

(4) Soumpasis, I., Grace, B. & Johnson, S. (2020). Real-life insights on menstrual cycles and ovulation using big data. Human Reproduction Open, 2020(2). 

(5) Bull, J. R., Rowland, S. P., Scherwitzl, E. B., Scherwitzl, R., Danielsson, K. G. & Harper, J. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. Npj Digital Medicine, 2(1).

(6) Mack, S.; Loytved, C. (2019). Wie genau ist das Schwangerschaftsalter zu bestimmen? Obstetrica, (8)2019, S. 8-11