Pregnancy Due Date Calculator

The due date is estimated by adding 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). This is based on Naegele's Rule — the standard method used by obstetricians worldwide. For example, if your LMP was 1 January, your estimated due date would be 8 October.

Naegele's Rule is the standard formula for estimating a pregnancy due date. It works by taking the first day of your LMP, subtracting 3 months, and adding 7 days. This is mathematically equivalent to adding 280 days (40 weeks) to the LMP. It assumes a regular 28-day cycle with ovulation on day 14.

A due date calculator provides an estimate, not a guarantee. Only about 5% of babies are born exactly on their due date. Most deliveries occur within 2 weeks before or after the estimated date. Accuracy improves when confirmed with a first-trimester ultrasound, which adjusts the date based on foetal measurements.

Gestational age is the number of weeks and days of pregnancy, counted from the first day of the LMP — not from the date of conception. At fertilisation, the gestational age is already considered to be about 2 weeks. A full-term pregnancy is 40 weeks of gestational age.

Pregnancy is divided into three trimesters:

• First trimester (Weeks 1-12): Major organ and brain development begins.
• Second trimester (Weeks 13-26): Rapid foetal growth; movements become noticeable.
• Third trimester (Weeks 27-40): Baby gains weight and prepares for birth.

Yes. If an early ultrasound (ideally before 14 weeks) shows foetal measurements that differ from the LMP-based estimate by more than 5-7 days, your doctor may revise the due date. First-trimester ultrasound is considered the most accurate method for confirming or adjusting the estimated due date.

Naegele's Rule assumes a regular 28-day cycle. If your cycle is longer, shorter, or irregular, the LMP-based estimate may be less accurate. In such cases, a first-trimester ultrasound is the most reliable way to establish gestational age and set an accurate due date. Always discuss this with your healthcare provider.

If you are unsure of your LMP, an early ultrasound can estimate gestational age based on foetal size and assign a due date. For IVF pregnancies, the due date is typically calculated from the egg retrieval or embryo transfer date, as the conception date is precisely known.