Pregnancy Due Date Calculator
Free pregnancy due date and gestational age calculator (Naegele's rule). Type the first day of your last menstrual period and a reference date to see your estimated due date.
Type the first day of your last menstrual period (LMP) and a reference date. The calculator returns the estimated due date (40 weeks from LMP) and your current gestational age.
Naegele’s rule
Due date = LMP + 280 days (= 40 weeks)
This is the formula obstetricians have used since the 19th century. It assumes a regular 28-day cycle with ovulation on day 14 — conception happens about two weeks after the LMP, but the 40-week clock starts at the LMP itself.
Gestational age vs fetal age
Two related but different counts:
- Gestational age — counted from the LMP. This is what doctors quote.
- Fetal age — counted from conception (gestational age − ~2 weeks).
A pregnancy “at 12 weeks” is at 12 weeks of gestational age. The embryo itself is about 10 weeks old.
How accurate is the date?
The “due date” is the estimated date of delivery (EDD) — a single day in a wide window. Roughly:
- Only ~4% of babies arrive exactly on the EDD.
- Around 80% are born between 38 and 42 weeks of gestation.
For women with irregular cycles, an early ultrasound (with crown-rump length measurement) is the more reliable dating method. This calculator just gives you the standard starting estimate.
Not medical advice
This is a quick arithmetic helper, not a substitute for prenatal care. Always discuss your pregnancy with a qualified healthcare provider.
Worked examples
-
LMP 2026-01-01, today the same — week 0, due Oct 8
Due 2026-10-08 — gestational age 0 weeks and 0 days.
-
Mid-pregnancy on 2026-05-21
Due 2026-10-08 — gestational age 20 weeks and 0 days.
Frequently asked questions
How is the due date estimated?
By Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). The rule assumes a 28-day cycle with ovulation on day 14, so conception is about two weeks after the LMP. The 40-week count starts at the LMP, not at conception.
How accurate is this estimate?
It's a starting estimate, not a prediction. Studies put only about 4% of births exactly on the due date; most occur within 3 weeks before or 2 weeks after. After the first ultrasound, providers usually re-date based on fetal measurements, which is more accurate for women with irregular cycles.
What if my cycle isn't 28 days?
Naegele's rule assumes 28 days. For longer / shorter cycles, your provider will adjust based on ovulation timing or, more reliably, the first-trimester ultrasound. Use this calculator as an approximate guide only.
This isn't medical advice, right?
Correct — it's the same simple arithmetic any clinician uses at the first visit, exposed here for your reference. Discuss your pregnancy with a qualified healthcare professional. Do not make medical decisions based on this calculator.
What's the difference between gestational age and fetal age?
Gestational age is counted from the LMP (so a baby "at 4 weeks" was technically conceived ~2 weeks ago). Fetal age, or post-conception age, is gestational age minus about 2 weeks. The clinical default — and what this calculator returns — is gestational age.
Related calculators
BMI Calculator
Free BMI calculator — type in weight and height to get your body mass index and the WHO weight category. Runs in your browser, no sign-up. Metric units (kg, cm).
EverydayDays Between Dates Calculator
Free days between dates calculator — type two dates and get the exact number of calendar days between them. Works in your browser, no sign-up.