Pregnancy typically lasts about 40 weeks, which equals roughly 10 months from the first day of the last menstrual period. However, it is often commonly referred to as nine months because pregnancy is usually counted from the missed period, about four weeks after conception, making the count approximately 36 weeks or nine months from that point onward.

Explanation of Pregnancy Length

• Pregnancy is medically measured as 40 weeks or 280 days from the first day of the last menstrual period, not from conception.
• Since conception usually occurs about two weeks after the start of the last period, actual fetal development lasts about 38 weeks.
• The 40 weeks correspond to about 10 calendar months, but because months vary in length (some are longer than 4 weeks), pregnancy is often colloquially stated as nine months.

Trimesters Breakdown

• Pregnancy is divided into three trimesters:
  • First trimester: Weeks 1 to 13 (about 3 months)
  • Second trimester: Weeks 14 to 27 (about 3.5 months)
  • Third trimester: Weeks 28 to 40 (about 3.5 months)

Thus, while pregnancy is about 10 months in duration overall, it is commonly discussed as lasting nine months for simplicity and conventional reasons.