Gestational Age Calculator
Enter your last period date or due date to find your gestational age, trimester, and upcoming milestones.
🤰 What is Gestational Age?
Gestational age is the standard measure of how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period (LMP). This convention — dating from LMP rather than from conception — has been used by obstetricians for over a century because LMP is a known, observable date, while ovulation and fertilisation can only be estimated.
At 40 weeks of gestational age, the pregnancy has reached its estimated due date (EDD). The EDD is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the LMP. The first trimester covers weeks 1 through 12, the second trimester weeks 13 through 27, and the third trimester from week 28 to birth. Most spontaneous deliveries occur between 38 and 42 weeks.
Gestational age is the reference used by all prenatal care guidelines, growth charts, and screening schedules. When your doctor says you are "20 weeks pregnant," this means 20 weeks from LMP — even though the embryo was only fertilised about 18 weeks ago. Ultrasound measurements confirm or revise gestational age by comparing actual fetal size to reference curves. An early ultrasound (8–13 weeks) measuring crown-rump length is accurate to within 5 days and is the gold standard for confirming gestational age when LMP is uncertain or irregular cycles are involved.
📐 Gestational Age Formula
From LMP (Naegele's Rule)
GA (days) = Today − LMP date
GA (weeks) = ⌊GA days ÷ 7⌋ Remaining days = GA days mod 7
Estimated Due Date (EDD) = LMP + 280 days
Estimated Conception Date = LMP + 14 days
From EDD
LMP = EDD − 280 days
GA (days) = Today − (EDD − 280)
Trimester Boundaries
1st Trimester: Weeks 1–12 | 2nd Trimester: Weeks 13–27 | 3rd Trimester: Weeks 28+
📖 How to Use This Calculator
Steps
1
Choose your mode — select "From LMP" if you know the date your last period started, or "From Due Date" if your doctor has given you an estimated due date (EDD).
2
Enter the date — type the LMP date or EDD in the date field. The date must be in the past for LMP mode (unless checking a future LMP); for EDD mode it should be a future date.
3
Click Calculate to see gestational age in exact weeks and days, your current trimester, estimated due date, conception date, and days remaining.
4
Check the next milestone — the calculator shows your next key pregnancy milestone (scan, screening test, trimester boundary) with approximate weeks remaining.
💡 Example Calculations
Example 1 — First trimester (LMP 10 weeks ago)
1
LMP = 10 weeks (70 days) ago from today (May 20, 2026) → LMP = Mar 11, 2026
2
GA = 70 days → 10 weeks 0 days. EDD = Mar 11 + 280 = Dec 15, 2026.
GA: 10 weeks 0 days | Trimester: 1st | EDD: Dec 15, 2026 | Next: Week 12 — nuchal translucency scan (in ~2 weeks)
Example 2 — Second trimester (LMP 20 weeks ago)
1
LMP = 140 days ago → Dec 31, 2025. GA = 140 ÷ 7 = 20 weeks 0 days.
2
EDD = Dec 31 + 280 = Oct 7, 2026. Days remaining = Oct 7 − May 20 = 140 days.
GA: 20 weeks 0 days | Trimester: 2nd | Next: Week 24 — viability milestone (in ~4 weeks)
Example 3 — Third trimester (from due date in 6 weeks)
1
EDD = July 1, 2026. LMP = July 1 − 280 days = Sep 24, 2025. Days elapsed = May 20 − Sep 24 = 238 days.
2
GA = 238 ÷ 7 = 34 weeks 0 days. Days remaining to EDD = July 1 − May 20 = 42 days.
GA: 34 weeks 0 days | Trimester: 3rd | Next: Week 37 — early term begins (in ~3 weeks)
❓ Frequently Asked Questions
How is gestational age calculated?
Gestational age 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 divided by 7); remaining days = days modulo 7. For example, if your LMP was 70 days ago, GA is 10 weeks 0 days. This calculator performs this automatically. If you only know your due date, it back-calculates LMP as EDD minus 280 days.
Why is gestational age measured from LMP and not conception?
Gestational age is measured from LMP because LMP is a known, observable date while conception (fertilisation) can only be estimated. This convention has been used by obstetricians for over a century. All prenatal reference charts, growth curves, and screening windows are calibrated to gestational age from LMP. The practical result is that gestational age is always about two weeks more than fetal (embryonic) age.
When do the trimesters start and end?
The three trimesters are: first trimester — weeks 1 through 12; second trimester — weeks 13 through 27; third trimester — weeks 28 to birth. Some sources define the boundaries slightly differently (e.g., first trimester as weeks 1–13), but the week-12 and week-28 markers are the most widely used in clinical obstetric practice and are used by this calculator.
How accurate is LMP-based gestational age dating?
LMP-based dating assumes ovulation on day 14 of a 28-day cycle and is most accurate for women with regular cycles. It can be off by several days to more than a week for women with irregular cycles or uncertain LMP recall. A first-trimester ultrasound measuring crown-rump length (CRL) is accurate to within 5–7 days and is preferred when LMP is uncertain or when CRL measurement differs from LMP dating by more than 5 days before 14 weeks.
What if I don't know my last menstrual period date?
If you do not know your LMP, use Mode 2 (From Due Date) 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 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.
What is the difference between gestational age and fetal age?
Gestational age counts from LMP; at 6 weeks gestational age, the LMP was 6 weeks ago, but fertilisation occurred only about 4 weeks ago. Fetal age (embryonic age) counts from conception — always approximately gestational age minus 2 weeks. Doctors, ultrasound reports, and all standard reference charts use gestational age. When you see "20 weeks pregnant," this means 20 weeks from LMP, roughly 18 weeks of fetal development.
What is crown-rump length (CRL) dating?
Crown-rump length (CRL) is the measurement from the top of the fetal head to the bottom of the torso, taken on first-trimester ultrasounds (8–13 weeks). Because early fetal size 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 table to derive gestational age and revise the estimated due date.
What does 40 weeks of pregnancy mean?
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 full-term pregnancy. 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 risks from placental aging.
What is a normal gestational age at birth?
Gestational age at birth categories: 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; post-term — 42 weeks or beyond. Preterm births carry higher neonatal risks, increasing with earlier gestational age. Extremely preterm (before 28 weeks) infants require intensive neonatal care.
How is gestational age used by my doctor?
Doctors use gestational age to schedule prenatal visits, interpret fetal growth on ultrasound, assess amniotic fluid levels, time corticosteroid administration for fetal lung maturity, and decide when to consider induction. Every prenatal test has a gestational-age window — nuchal translucency is measured between 11 and 14 weeks, and the anatomy scan at 18–22 weeks.
Can gestational age differ from what my ultrasound says?
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. After 14 weeks, ultrasound dating becomes less precise and LMP-based dates are typically kept unless there is a large discrepancy.
Does gestational age change if I have an irregular cycle?
LMP-based gestational age assumes a 28-day cycle with ovulation on day 14. If your cycle is longer — say, 35 days — ovulation likely occurred around day 21, meaning fertilisation was about 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 rather than relying on the LMP date alone.