How long does a pregnancy last?

Eva Rudolf-Müller is a freelance writer in the medical team. She studied human medicine and newspaper sciences and has repeatedly worked in both areas - as a doctor in the clinic, as a reviewer, and as a medical journalist for various specialist journals. She is currently working in online journalism, where a wide range of medicine is offered to everyone.

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Nine months or ten months? When it becomes known, pregnancy often first raises the question of when the baby will be born. There are different calculation methods for the possible delivery date. The last menstrual period is usually taken as the basis. Read more about the different methods of calculating pregnancy and its duration here.

Pregnancy: calculated according to the menstrual period

Most women do not know the exact date of conception, but they do know the first day of their last menstrual period. On this basis, the duration of the pregnancy can be calculated using the so-called Naegele rule: With a regular 28-day cycle, seven days and a year are added from the first day of the last menstrual period and then three months are deducted. The result counts as the possible delivery date. Example:

Last period on 05/30/2019 + 7 days + 1 year = 06/06/2020 - 3 months = 06/03/2020

According to the Naegele rule, a pregnancy lasts arithmetically 280 days (40 weeks).

But how long does a pregnancy last if the cycle is irregular? In such a case, the days that deviate from the 28-day cycle on average are added to or subtracted from this calculation. This so-called extended Naegele rule is most often used to calculate the duration during pregnancy.

Pregnancy: calculated according to the time of conception

If, on the other hand, you know the time when conception took place, the calculation is easier: seven days and three months are deducted from the conception date and one year is added. This results in a calculated gestation period of 267 days.

When neither conception nor last menstrual period are known

In rare cases the cycle information is extremely imprecise. If the time of conception is also unknown, none of the above calculation methods can be used. The first occurrence of child movements is then taken as an indication: first-time mothers notice the first movements around the 20th week of pregnancy, second or multiple births as early as the 18th week of pregnancy. With deviations of two weeks earlier or later, these child movements give an indication of the duration. In such cases, pregnancy also means that one has to live with major temporal inaccuracies.

Calculation based on the ultrasound

A very precise determination of the age of the child is now possible with an ultrasound examination. Because in the first few weeks, the size of the embryo is closely related to the age of pregnancy. The diameter of the amniotic sac can be measured from the seventh week, and the first heartbeat of the embryo can be heard as early as six weeks after menstruation.

How is the duration specified?

Pregnancy is given in completed weeks and expired days of the current week. For example, if you are 36 weeks and three days pregnant, i.e. in the 37th week, your doctor will write: 36 + 3SSW - this is what it says in the maternity record.

When is the baby actually coming?

The percentage of babies who are actually born on the calculated due date after 267 or 280 days is just four percent. 30 percent of the children come four days earlier or later and 66 percent within three weeks (with a tolerance of ten days) before or after the calculated date. You should keep this in mind when looking at the calculated duration of pregnancy in the maternity record.

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