Pregnancy: calculation after menstruation
Most women do not know the exact date of conception, but the first day of the last menstrual period. On this basis, the duration of pregnancy can be calculated using the so-called Naegele rule: For a regular cycle of 28 days, seven days and one year are added from the first day of the last menstrual period and then three months are subtracted. The result is considered the possible date of delivery. Example:
According to the Naegele rule, a pregnancy thus lasts 280 days (40 weeks) in purely mathematical terms.
But how long does a pregnancy last if the cycle is irregular? In such a case, the average days deviating from the 28-day cycle are added to or subtracted from this calculation. This so-called extended Naegele rule is most often used to calculate the duration in pregnancy.
Pregnancy: Calculation according to the time of conception
If neither conception nor last menstrual period are known
Calculation based on the ultrasound
A fairly accurate determination of the age of the child is possible today with an ultrasound examination. This is because in the first weeks the size of the embryo is closely related to the gestational age. The diameter of the amniotic sac can be measured from the seventh week, and the first heart sounds of the embryo can be heard as early as six weeks after menstruation.
How is the duration indicated?
When will the baby actually be born?
The percentage of babies who are actually born on the calculated due date after 267 or 280 days is just four percent. Thirty percent of babies are born 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 log.
