Gestational Age

Gestational age: counting weeks from LMP

Gestational age is counted from the first day of the last menstrual period (LMP), not from conception — which occurs roughly two weeks later, around ovulation. Term pregnancy is 40 weeks (280 days) from LMP; the estimated due date (EDD) follows Naegele's rule: LMP + 7 days − 3 months + 1 year. First-trimester ultrasound measures the crown-rump length (CRL) and is the most accurate dating method; if it differs from LMP by more than 5 to 7 days, the ultrasound date replaces LMP. Example: LMP on Jan 15, reference date Mar 26 → 70 days = 10 weeks and 0 days; EDD on Oct 22.

Applications: prenatal care in Brazil

Brazil's SUS prenatal protocol (PNAB, Rede Cegonha) recommends at least 6 consultations: 1 in the first trimester, 2 in the second, 3 in the third. Key milestones: morphological ultrasound at 11–13 weeks + 6 days (nuchal translucency screening for chromosomal anomalies), second-trimester scan at 20–24 weeks (fetal anatomy), and biophysical profile in the third trimester. Gestational age also guides drug safety by trimester — for example, folic acid before 12 weeks reduces neural-tube defects.

FAQ

Why count from LMP and not conception? LMP is a date the patient usually remembers; ovulation and fertilization are not directly observable. The 2-week offset is built into the 40-week term.

What if my cycle is irregular? First-trimester ultrasound (CRL) becomes the reference. After 14 weeks, dating accuracy drops.

Is 37 weeks already term? 37–38+6 is "early term", 39–40+6 is "full term", 41+ is "late term", and 42+ is post-term. Elective deliveries before 39 weeks without medical indication are discouraged.

How accurate is the EDD? Only about 4% of births happen exactly on the EDD; most occur within a two-week window around it.