# Due date calculation

Due Date ﹣ Calculator

Determine your expected due date using the first day of your last period and the mean duration of the cycle with our Due Date Calculator. The output of many other data, such as fertilization date and dates of the child's developmental phases make the calculator a comprehensive information medium for expectant parents.

## Necessary data for the calculation of the due date

The calculator requires the following two inputs to determine the expected date of birth:

### Date of birth: What do you need the first day of your last period for?

Using this data and the duration of your cycle, the date of your last ovulation can be determined. This gives us the time of fertilization, i.e., when the sperm cell enters the egg cell. From this point on, a pregnancy l asts an average of 266 days or 38 weeks. With this, the expected date of birth can be calculated on the basis of the data entered. Fertilization occurs 14 days after the start of the last menstrual period, assuming 28 days. If the duration of the cycle involves more or less days, an adjustment of the calculation is necessary.

Since the exact time of fertilization is usually unknown, the duration of a pregnancy is measured in medicine from the beginning of the last menstrual period. Therefore, the defined duration of a pregnancy is 14 days + 38 weeks = 40 weeks of pregnancy.

### Date of birth: What role does the length of the cycle play?

The cycle lasts from the first day of menstruation up to and including the last day before the next menstruation. Using the cycle and the beginning of the last menstrual period, the date of fertilization (conception) can be determined. Together with the average duration of pregnancy of 38 weeks from fertilization, the date of birth can then be calculated on the basis of the week of pregnancy.

Generally, ovulation and thus possible fertilization takes place 14 days before the start of the next period. With a cycle of 28 days, possible fertilization takes place 14 days after the first day of the last bleeding. With a cycle of 32 days, for example, the time from ovulation to the next menstruation is 14 days. However, it takes in this case 32 - 14 = 18 days from the start of the last bleeding to possible fertilization. Therefore, the cycle length for calculation of the birth date is also taken into account: The calculated due date for a pregnant woman with a 28-day cycle is 40 weeks after the start of the last menstrual period (38 weeks from fertilization plus 14 days). On the other hand, with a 32-day cycle, 4 days more are calculated, since fertilization is assumed to take place 18 days after the start of the last bleeding.

## What is the formula for calculating the date of birth?

The expected date of birth is determined according to the following formula:

Date of birth = First day of last period + (duration of cycle - 14) + 266 days

## The calculated date of birth

The calculated due date is a 28-day cycle, which is exactly 40 weeks after the first day of the last period. Around 4 percent of children are born exactly on the calculated due date. Around the date within one week, 26 percent are born and around the delivery date within three weeks, 66 percent are born. Around 90 percent of all children are born between the 37th and 42nd week of pregnancy. They are called "term babies". About nine percent are born before this period, which means they are premature. Meanwhile, only about one percent of children are born after this. These late births are called post-term. It should be noted that the natural course of the pregnancy is usually influenced here to prevent transmission.

## What is the relationship between the due date and the week of pregnancy?

The term "week of pregnancy" is certainly familiar to every pregnant woman. The pregnancy week mark often relevant events during pregnancy. For example, when do the organs develop, or the hearing, or when does the child move for the first time? Feel free to use our Pregnancy Week Calculator to find out your current week of pregnancy. The calculated date of birth or due date is exactly 40 weeks after the first day of the last period in a 28-day cycle. As the individual time periods slightly vary from woman to woman, the gynaecologist may still correct the expected date of birth in the course of the pregnancy.

## Source information

As source for the information in the 'Due Date' category, we have used in particular:

## Last update

This page of the 'Due Date' category was last edited or reviewed by Stefan Banse on February 19, 2023. It corresponds to the current status.