Gestational Age Calculator

Gestational age (GA) is calculated from the first day of the last menstrual period (LMP) to the current date: GA in days = Today minus LMP; GA in weeks = floor(days ÷ 7); remaining days = days mod 7. For example, if your LMP was 70 days ago, your GA is 10 weeks 0 days. This calculator performs this arithmetic automatically. If you only know your due date, it back-calculates LMP as EDD minus 280 days.

Gestational age has been measured from LMP for over a century because LMP is a known, observable date while conception (ovulation + fertilisation) can only be estimated. The LMP convention is used universally by obstetricians, midwives, and ultrasound standards — so all reference charts, milestone tables, and growth curves are calibrated to gestational age, not fetal age. The practical result is that gestational age is always about 2 weeks more than fetal age.

There are three trimesters: the first trimester covers weeks 1 through 12, the second trimester covers weeks 13 through 27, and the third trimester runs from week 28 to birth. Some sources define the boundary differently (e.g., first trimester as weeks 1–13), but the week-12 and week-28 markers are the most widely used in clinical practice. This calculator uses weeks 1–12 for the first trimester, 13–27 for the second, and 28+ for the third.

LMP-based dating assumes ovulation on day 14 of a 28-day cycle and is most accurate for women with regular 28-day cycles. It can be off by several days to a week or more for women with irregular cycles, long or short cycles, or uncertain LMP recall. An early ultrasound (before 14 weeks) measuring crown-rump length (CRL) is accurate to within 5–7 days and is preferred when the LMP is uncertain or when CRL measurement differs from LMP dating by more than 5 days in the first trimester.

If you do not know your LMP, use Mode 2 (From Due Date) in this calculator if you have an estimated due date from an ultrasound. If you have neither, an ultrasound scan is the only reliable way to determine gestational age. At 8–10 weeks, crown-rump length (CRL) dating is accurate to within 3–5 days. At 14–20 weeks, head circumference and femur length measurements are accurate to within 7–10 days.

Gestational age counts from LMP: at 6 weeks gestational age, LMP was 6 weeks ago, but fertilisation occurred only about 4 weeks ago. Fetal age (embryonic age) counts from conception — so fetal age is always approximately gestational age minus 2 weeks. Doctors, ultrasound reports, and all standard reference charts use gestational age. When you see '20 weeks pregnant,' that means 20 weeks from LMP, or approximately 18 weeks of fetal development.

Crown-rump length (CRL) is the measurement from the top of the fetal head to the bottom of the torso, used on first-trimester ultrasounds (typically 8–13 weeks). Because fetal size in the first trimester varies very little between individuals of the same gestational age, CRL is the most accurate dating method available — typically within 5 days. The sonographer measures CRL and uses a standardised lookup table to derive gestational age and revise the estimated due date.

Forty weeks of gestational age is the standard estimated due date (EDD), calculated as LMP plus 280 days (Naegele's rule). It represents the expected endpoint of a full-term pregnancy and corresponds to approximately 9 months and 1 week from the first day of the last period. Most spontaneous deliveries occur between 38 and 42 weeks. A pregnancy reaching 42 weeks (post-term) is typically managed with induction or close monitoring due to increased placental aging risk.

Gestational age at birth is categorised as: preterm (before 37 weeks), early term (37 weeks 0 days to 38 weeks 6 days), full term (39 weeks 0 days to 40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days), and post-term (42 weeks or beyond). Premature births (before 37 weeks) carry higher neonatal risks, with severity increasing with earlier gestational age. Extremely preterm (before 28 weeks) infants require intensive neonatal care.

Doctors use gestational age to schedule prenatal visits and screening tests, interpret fetal growth on ultrasound, assess amniotic fluid levels, time corticosteroid administration for fetal lung maturity, and decide when to consider induction or caesarean delivery. Every prenatal test has a window defined in gestational weeks — for example, nuchal translucency is measured between 11 and 14 weeks, and the anatomy scan is performed at 18–22 weeks.

Yes, discrepancies between LMP-based dating and ultrasound dating are common. If the first-trimester ultrasound (before 14 weeks) shows a gestational age that differs from LMP dating by more than 5–7 days, the ultrasound estimate is usually adopted as the basis for the due date. After 14 weeks, ultrasound dating becomes less precise (window widens to 10–14 days) and LMP-based dates are typically kept unless there is a large discrepancy.

Key gestational milestones: week 6 — heartbeat detectable by transvaginal ultrasound; week 8 — embryo officially becomes a fetus; week 10–13 — first-trimester combined screening (nuchal translucency + blood tests); week 12 — end of first trimester; week 20 — anatomy scan; week 24 — viability milestone (most NICUs can support survival); week 28 — start of third trimester; week 37 — early term; week 39 — full term begins; week 40 — estimated due date; week 42 — post-term.

LMP-based gestational age assumes a 28-day cycle with ovulation on day 14. If your cycle is longer (e.g., 35 days), ovulation likely occurred on day 21, meaning fertilisation was a week later than in a 28-day cycle. The true due date may be 7 days later than LMP dating suggests. A first-trimester ultrasound corrects for this by measuring actual fetal size. If you know your ovulation date or have a confirmed conception date, using it (adding 266 days) gives a better estimate than LMP alone.