Pregnancy Calculator
Answer: pregnancy lasts about 40 weeks (280 days) from the first day of the last period. Pick your method below and get your due date, current week and day, trimester, estimated baby size, and upcoming milestones — instantly and free. (ACOG dating guidance)
- 1Method
- 2Details
- 3Result
Pick the date
Which dating method should you use?
| Method | Best for | Formula |
|---|---|---|
| Last period | Regular cycles | LMP + 280 days ± cycle correction |
| Conception / IUI | Known ovulation day | Date + 266 days |
| IVF day 5 / day 3 | Embryo transfer | Transfer + 261 / 263 days |
| Frozen transfer | Known freezing age | Theoretical LMP + 280 days |
| Ultrasound | First-trimester scan | Completes 280 days from measured age |
All five formulas follow standard obstetric dating. (ACOG)
How accurate is it?
Only a small share of babies arrive exactly on the estimated date; the normal range spans weeks 37 to 42. Irregular cycles shift LMP-based dates by days, which is why the tool asks for your cycle length — and why a first-trimester ultrasound remains the reference for correcting the estimate. (ACOG)
The three trimesters at a glance
Pregnancy week by week
Each week of the result links to a familiar size comparison — from a poppy seed in week 4 to a watermelon near week 40 — drawn from a medically reviewed week-by-week size guide. (BabyCenter, reviewed by an embryologist) For what happens inside each week, the UK health service publishes a full week-by-week pregnancy guide. (NHS) Mayo Clinic’s week-by-week program answers the everyday questions — food, exercise, tests — as the months go by. (Mayo Clinic)
Gentle natural care during pregnancy
Balanced nutrition, rest, and regular antenatal check-ups are the foundation of a healthy pregnancy according to global health guidance. (WHO) For skin elasticity and comfort, many mothers turn to plain plant oils — always check each ingredient with your doctor first, because “natural” does not automatically mean safe in pregnancy. This page is an estimate, not medical advice: confirm every date and every product with a qualified professional.
Frequently asked questions
How long is pregnancy?
Pregnancy lasts about 40 weeks (280 days) counted from the first day of the last menstrual period, though only a minority of babies arrive exactly on the due date (American College of Obstetricians and Gynecologists).
How many weeks is 9 months pregnant?
Nine calendar months land around weeks 36 to 40, so a pregnancy described as “9 months” is usually in the final stretch before the estimated due date.
How is the due date calculated from IVF?
Add 261 days to a day-5 embryo transfer date, or 263 days for a day-3 transfer — the embryo’s age at transfer sets the gestational age (American College of Obstetricians and Gynecologists).
When should I take a pregnancy test?
Home tests are most reliable from the first day of a missed period, roughly 14 days after ovulation; testing earlier can give a false negative (UK National Health Service).
What is the difference between gestational age and fetal age?
Gestational age counts from the last period and is about two weeks longer than fetal age, which counts from conception — doctors use gestational age for all milestones.
How is the due date calculated from ovulation?
Add 266 days (38 weeks) to the ovulation or conception date, since fertilization happens about two weeks after the last period in a 28-day cycle.
How accurate are due-date calculators?
LMP-based dates shift with irregular cycles; a first-trimester ultrasound is the most accurate dating method and can correct the estimate by several days (American College of Obstetricians and Gynecologists).
Estimates only — not medical advice. Confirm your dates with your doctor or midwife. Last reviewed 2026-09-20.