How is pregnancy length calculated from the last menstrual period and from conception?

In human biology, pregnancy duration is typically calculated using two different starting points, which often leads to confusion.

Calculation from the Last Menstrual Period (LMP)

Gestational age is most commonly measured from the first day of the mother’s last menstrual period. Physicians often estimate the date of delivery by adding seven days to the first day of the LMP and counting forward nine calendar months. On average, this method results in a full-term pregnancy of approximately 280 days (40 weeks). Because the exact date of ovulation is rarely known, this calculation serves as a standard clinical estimate rather than a precise measurement of fetal age.123

Calculation from Conception

The actual age of the fetus—often referred to as the duration of pregnancy—is measured from the time of conception, which occurs at ovulation. There are typically 266 to 270 days between ovulation and childbirth, representing a developmental age of about 38 weeks.31

While 40 weeks (from LMP) is considered full-term, individual pregnancies vary significantly, and there is only a 5 percent chance that a baby will be born on the exact date estimated by the LMP rule.12