How to calculate your estimated due date (DPP)

How to calculate your estimated due date (DPP)

Due date is calculated by adding 40 weeks (or 280 days) to the date on which your last period started, or by adding 38 weeks (or 266 days) to the date you conceived.

But what is the reason for this count? And are there any methods to simplify the calculation? Let’s see it together.

How the calculation works

Human gestation lasts approximately 38 weeks (or 266 days) from the date of conception . 

However, unless you know exactly when you ovulated, it’s very difficult to know exactly when your baby was conceived, even if you’ve only had intercourse once during your fertile window .

Fertilization, in fact, can also take place at a certain distance from sexual intercourse, given that sperm can live up to 5 days inside the female body.

For this reason, gynecologists calculate the date of delivery by taking as reference the first day of the last menstrual period (which represents a certain date) and considering a duration of pregnancy of 40 weeks (or 280 days) .

Of course, this is a conventional calculation, based on the fact that a woman’s cycle typically lasts 28 days and ovulation occurs on the 14th day on average . Consequently, it is highly probable that conception occurred 2 weeks after the start of the menstrual cycle (38+2=40).

This method, however, does not account for many variables, such as whether many women have cycles shorter or longer than 28 days. Also for this reason, gynecologists refer to the date of birth of the future baby with the term Presumptive Delivery Date (DPP) , to underline the fact that it is only an approximate calculation.

In fact, remember that only 30% of pregnancies end after 40 weeks, while most women give birth before or after the expected deadline .

Notes on calculating the expected date of delivery

The calculation made with the obstetric ruler (and therefore with our calculator) is reliable only if the expectant mother is sure of the start date of her last menstrual period, if her flow was normal with reference to her usual characteristics and if the menstrual cycle does not exceed an average of 35 days in duration.

Furthermore, the calculation could be inaccurate even if hormonal contraceptives (such as the pill ) were used in the 3 months preceding the last menstruation.

In any case, within the first 11 weeks of pregnancy the gynecologist will carry out the first ultrasound ( ultrasound for dating ), which will allow to measure the baby and to date the pregnancy with greater precision.

Table to calculate the expected date of delivery

If you know the start day of your last menstrual period, you can calculate your due date with a good approximation using the following table .

Look for the date of the last period (example March 10) in the colored lines: in the line below you will find the expected date of birth (example December 15).

Formula for calculating the expected due date

If you like math, you can also calculate DPP with this formula :

• subtract 3 months from the first day of your last period
• add 7 days
• add 1 year

Example

If your last period started on 5/10/2022, subtract three months and you get 2/10/2022. Adding seven days results in 2/17/2022. Finally, adding a year you will get 2/17/2023 as the expected date of birth.

How to calculate the expected date of birth from conception

If you know the date of conception , for example because you are monitoring ovulation with the appropriate sticks or with other methods ( basal temperature or cervical mucus ), you can calculate the due date by adding 38 weeks (or 266 days).

Alternatively, to use the calculator, table, or formula above, subtract 14 days and use the result as your period start date.

Example

If conception took place on May 24, 2022, subtracting 14 days will give May 10, which represents the date to enter in the calculator or to use for the calculation with the table or formula described above.

You can also use this method in the case of IVF, considering the transfer date as the day of conception .

What is the use of calculating the expected due date?

If the probability of giving birth on the expected day is so low, it would be natural to ask why calculate the expected date of delivery .

In reality, it is a fundamental calculation, which serves to establish the level of prematurity of the baby at birth.

According to the World Health Organization (WHO), in fact, we are talking about:

–     preterm pregnancy , when the birth occurs at less than 37 weeks, i.e. less than 259 days of gestation (premature birth)

–     term pregnancy , when the birth takes place between the beginning of the 37th ^week , (37+0) up to the complete 41st ⁽ 40+6), i.e. from 259 to 293 days;

–     pregnancy beyond the term , from the beginning of the 42nd ^week (41+0) onwards, ie from 294 days onwards.

By calculating the DPP, therefore, it is possible to establish which category the pregnancy falls into and to act accordingly.

Specifically, in the event of  a preterm birth , it will be possible to plan the most suitable treatments for the child, while in the event of  a pregnancy beyond the term, it will be possible to decide whether and when to induce labor artificially .