Duration of pregnancy
The duration of a pregnancy can be calculated using a simple formula.
For women whose cycle has a length of 28 days, the pregnancy lasts 40 weeks.
These are counted from the 1st day of the last menstruation, or 10 lunar months of 28 days each.
From the moment of conception (fertilization or conception), pregnancy lasts 267 days or 38 weeks.
Calculation of the date of birth
Basis for the computation of the date of birth is the so-called "naegelesche rule",
which goes back to researches of the Heidelberger Gynäkologen Franz Naegele (1778-1851).
A cycle length of 28 days is taken as a basis and calculated with the following formula:
Date of birth =
[1st day of last period] + 7 days - 3 months + 1 year
For women whose cycle length does not correspond to the standard 28 days,
the extended naegelesche rule is applied:
Date of birth =
[First day of last period] + 7 days - 3 months + 1 year ± [days difference]
With this formula you can calculate the approximate date of birth of the baby.
However, you should keep in mind that a final confirmation can only be made by the gynecologist
in the course of a comprehensive examination!
With the help of the ultrasound examination at the beginning of the pregnancy it can be determined more precisely in which week of pregnancy you are.
Actual date of birth
You should be aware that you can only determine an approximate date using the calculation
- the calculated date is not necessarily the actual date of birth!
Statistically speaking, you have to assume a deviation of ten days around the expected date of birth
- i.e. 68% of all babies are born in a period of ten days before to ten days after the calculated date and
only 4% on the calculated date! Therefore, you should take such a possible deviation into account when planning your birth.
I will gladly answer any further questions in my courses!