How to calculate your due date

Pregnancy is a process that starts immediately after fertilisation. The average length of pregnancy depends on various factors, which can be environmental and genetic. You may need an exact or approximate due date to make a birth list, welcome the unborn baby in good conditions and prepare. Based on the date of the last menstrual period, multiple methods exist for calculating your due date.

Calculating the delivery date: what process?

Doctors mainly use ultrasound to calculate the due date. However, this process does not always consider late reports.

The survey

Even if the ultrasound and other clinical data are available, the specialist must first ask the pregnant woman several questions. The answers obtained also help determine the delivery date.


One of the first pieces of information the doctor needs is the exact date of the last period. However, for this information to be useful, the date of the last period must be related to the woman's usual cycle. Thus, it is difficult to theorise about the delivery date when the woman's cycles are irregular. With regular cycles, 12 or 16 days should be added to the date of the last menstrual period depending on the type of cycle (long or short). This method is generally attributed to Naegele, who stated that the average duration of a pregnancy is 40 weeks and that 280 days should be added to the date of the last period. However, not all specialists agree with this method.


Ultrasound technology has evolved considerably, and the results constantly exceed expectations. Thus, it is also used as a reference for a pregnancy. The ultrasound dating test provides an estimated delivery date when performed in the first half of the year. However, this may be delayed by a few days.

What other methods exist for calculating my due date?

Many other methods exist for determining the delivery date besides the Naegele method.

Wood's method

Wood's method considers the number of pregnancies a woman has had in her lifetime and the length of her menstrual cycle to determine the exact delivery date. Thus, the estimates differ between a woman having her first pregnancy and one who has had several. In the first case (according to this formula), add 12 months to the date of the last period and subtract 2 months and 2 weeks from the total. In the second case, the same formula should be used, but 2.5 weeks should be deducted this time.

The Mittendorf and William formula

The Mittendorf and William formula is as well-known as the Naegele formula. According to several studies, it is also extremely effective in determining the delivery date. It states that when women give birth for the first time, the pregnancy is slightly longer than for the second time. The number of days can therefore vary between 283 and 288 days from their last period. Mittendorf and William's formula can therefore be stated as follows: 1st day of last period + 12 months − 3 months + 15 days for women giving birth for the first time. For other cases, the formula is as follows: 1st day of last period + 12 months − 3 months + 10 days.

The Parikh method

The Parikh method targets women whose menstrual cycle is irregular. The expected delivery date is calculated by adding 9 months to the date of the last menstrual period and subtracting 21 days, considering the length of the previous cycles. This method should not be used for women whose menstrual cycle has always been regular.

What is the margin of error in calculating the exact delivery date?

The various methods and formulas mentioned above may be confirmed; however, this does not mean that they are infallible. While the date of the last menstrual period should be used as the basis for determining the delivery date, it should also be remembered that margins of error exist, and your estimated date of fertilisation is not always correct. The length of a cycle can vary from one woman to another, and variations can occur within the same woman. Many people may be unaware that other factors exist, such as the date of ovulation and the life span of the sperm.

Duration of pregnancy: why should we put it into perspective?

As mentioned above, the date of the last menstrual period, the length of the cycle and the number of pregnancies must all be considered. Although it is commonly said that the duration of a pregnancy is 9 months, this information is not always sufficient to determine the date of pregnancy. It should also be remembered that the date of fertilisation could be used to determine the date of pregnancy and obtain even more accurate results. Unfortunately, not all women are aware of their ovulation.

Why is it important to know your due date?

By making an appointment with the gynaecologist, you can estimate the pregnancy date, arrange appropriate follow-up and prepare for the baby's arrival. Moreover, this delivery date is the reference date for establishing close medical monitoring if the term is exceeded and, if necessary, for inducing delivery if it has still not occurred after 42 completed weeks of amenorrhea  (i.e. 295 days of pregnancy).