Conception usually occurs two weeks after the start of the next menstrual cycle. To calculate the gestational age and calculate the approximate date of birth, obstetricians count 40 weeks from the first day of the last menstrual period.
To maintain a pregnancy calendar, obstetricians are guided by just such a reference system, in which the weeks are called “obstetric” and “advance” the actual pregnancy by 14 days. The actual pregnancy occurs on the date of conception.
The reason why obstetricians use their own scale for calculating the duration of pregnancy is simple: due to the fact that the cycle of different representatives of the fair half of humanity can vary greatly in duration, it can be difficult to determine the exact date of conception. Calculating the timing of pregnancy and the date of birth from the first day of the last menstruation in this case is a more reliable method.
You can calculate your expected due date yourself. Use the Naegele formula (F. K. Naegele): from the first day of the last menstruation, count 3 months ago and add 7 days. Any pregnancy calculator you find online will be based on this exact formula.
In the future, the physical development of your unborn child, data about which can be obtained using a number of studies (including ultrasound), may make slight adjustments in determining the date of birth, but obstetricians’ forecasts are sometimes surprisingly accurate.