Your pregnancy due date, calculated four ways

Estimated due date

Thursday 17 December 2026

224 days to go · Baby's zodiac: Sagittarius

The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter, this calculator adjusts accordingly. Doctors count pregnancy in "gestational weeks" — measured from the LMP, not from conception. This means at the moment of conception (around week 2–3), you're already considered 2–3 weeks pregnant by medical reckoning. The full term of 40 weeks equals roughly 9 months and 1 week.

Due date calculators give an estimate. Only about 4–5% of babies are born exactly on their due date. Most births occur within 2 weeks before or after — anywhere from 38–42 weeks is considered "normal term." The most accurate method for determining a due date is an early ultrasound (before 14 weeks), which measures the size of the embryo/fetus (crown-rump length). If your LMP-based date and ultrasound date differ by more than 5–7 days, your doctor will typically use the ultrasound date.

For IVF pregnancies, the due date is calculated from the embryo's fertilisation date, not the transfer date. For a day-5 blastocyst: subtract 5 days from the transfer date to find fertilisation, then add 266 days (38 weeks). For day-3 embryos: subtract 3 days, then add 266 days. This calculator handles both automatically. Your IVF clinic will typically provide the due date, but this calculator lets you verify it and see your milestone schedule. IVF pregnancies follow the same gestational development timeline as natural pregnancies.

Pregnancy is divided into three trimesters: First trimester (weeks 1–12): Major organ development occurs. Risk of miscarriage is highest. Morning sickness is common. The nuchal translucency scan typically occurs at 11–13 weeks. Second trimester (weeks 13–27): Often called the "honeymoon trimester" — energy returns, morning sickness eases. The 20-week anatomy scan checks fetal development. You'll feel movement (quickening) around weeks 16–22. Third trimester (weeks 28–40+): Rapid growth phase. The baby's position becomes important. Signs of labour to watch for include regular contractions, waters breaking, and reduced fetal movement (which warrants immediate medical attention).

Gestational age (GA) is counted from the first day of your last menstrual period — this is how doctors and midwives measure pregnancy. At 4 weeks gestational age, conception has typically just occurred. Foetal age (or embryonic age) is measured from fertilisation. It's always approximately 2 weeks less than gestational age. Medical dating uses gestational age exclusively, so when your doctor says "you're 12 weeks pregnant," that means 12 weeks from your LMP, not from conception.