In more detail
Gestational age is counted from the first day of the last menstrual period (LMP), so it includes about two weeks before conception. Naegele's rule gives the due date as LMP + 280 days (or add one year, subtract three months and add seven days); it assumes a 28-day cycle with ovulation on day 14, so longer or shorter cycles shift it. Ages are written as weeks plus days, for example 31+4. A first-trimester ultrasound measurement is the most accurate way to date a pregnancy and replaces the LMP date when the two differ by more than a set margin. The trimesters begin at 0, 14 and 28 weeks. See How Pregnancy Due Dates Are Calculated.
Why it matters
Screening tests and scans are timed by gestational age, so wrong dating can make normal results look abnormal. Births from 37 weeks are term (39+0 to 40+6 is "full term"); before 37 weeks is preterm and from 42 weeks post-term. Only a small fraction of babies arrive on the due date itself. Dating and care decisions belong to a midwife or doctor.
Example
LMP 1 January 2026 + 280 days → due 8 October 2026
LMP 2 March 2026, on 9 October 2026: 221 days = 31 weeks 4 days (31+4)
→ third trimester, due 7 December 2026
Common confusion
Fetal (conceptional) age counts from conception and is about two weeks less: at 38 weeks' gestation the fetus is about 36 weeks old. For conception timing see Ovulation.