
Childbirth usually occurs about 38 weeks after fertilization (conception), i.e., approximately 40 weeks from the last normal menstrual period (LNMP) in humans. The date of delivery is considered normal medically if it falls within two weeks of the calculated date. The calculation of this date involves the assumption of a regular 28-day period. Thus, pregnancy lasts almost nine months.
The exact definition of the English word “pregnancy” is a subject of political controversy, but it is not a matter of substantial controversy in the medical community.