It all depends on what is considered a reference point. Physically, pregnancy occurs at the moment of conception. However, medical professionals consider it appropriate to count the period starting from the first day of the last menstrual cycle (about 2 weeks earlier). The last cycle is considered, after which the delay of menstruation begins.
On average, a normal pregnancy lasts 266 days (38 weeks) from conception and 280 days (40 weeks) from the first day of the last menstrual cycle.
Stop! That's 10 months! How can this be?
If you divide 38 weeks by 4, you get 9.5 months. 40 weeks is actually 10 months. However, by dividing the term by 4, you are calculating lunar months that are shorter than calendar months.
The duration of the lunar month is 28 days, that is, four 7-day weeks. A calendar month is 30-31 days long. Therefore, the gestational age, calculated in lunar months, seems longer than it really is. The 40 week period includes 2 weeks before conception.
Thus, depending on the calculation method, pregnancy lasts 38-40 weeks, or 9 calendar months.