Due Date Calculator

How it works: Naegele's rule

A typical pregnancy lasts about 280 days (40 weeks) counted from the first day of the last period — not from conception, which usually happens about two weeks later. Naegele's rule is a shortcut for the same thing:

Due date = LMP + 1 year − 3 months + 7 days (= LMP + 280 days)

The rule assumes a 28-day cycle with ovulation on about day 14. If your cycles are longer or shorter, ovulation probably happened later or earlier, so we shift the date by the difference: a 32-day cycle adds 4 days, and a 26-day cycle subtracts 2.

Gestational age and trimesters

Gestational age is counted in weeks and days from the LMP. By convention, the first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. ACOG defines "full term" as 39 weeks 0 days to 40 weeks 6 days; 37–38 weeks is "early term" and 41 weeks is "late term."

Why your provider's date may differ

ACOG recommends that the due date be confirmed with an ultrasound in the first trimester, which measures the embryo or fetus directly and is the most accurate way to date a pregnancy. If the ultrasound date differs from the LMP date by more than a set margin (for example, more than 7 days before 14 weeks), the ultrasound date is generally used. For pregnancies conceived with IVF, the due date is calculated from the embryo transfer date instead. Once your provider sets a due date, follow theirs.

Only an estimate

Very few babies arrive exactly on their due date; most are born within about two weeks either side. Think of the date as the middle of a window rather than an appointment.

If your cycle isn't 28 days

Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is consistently longer or shorter, the calculator shifts the due date by the difference, and the same shift is applied to gestational age and trimester dates shown in the result. For example, a 35-day cycle moves the date 7 days later. This adjustment is only an approximation: ovulation can vary within the same person, and providers usually rely on an early ultrasound when cycles are irregular or the LMP is uncertain.

Key pregnancy milestones

Timing varies by country and provider.

When to call your provider

Contact your midwife, doctor or maternity unit promptly — or emergency services if severe — if you have vaginal bleeding, severe or persistent abdominal pain, a severe headache or vision changes, sudden swelling of the face or hands, fluid leaking from the vagina, fever, or (later in pregnancy) your baby moving less than usual. Trust your instincts: it is always fine to call. See the NHS guide to bleeding in pregnancy and ACOG's Bleeding During Pregnancy FAQ.

Twins and other situations

Twin and multiple pregnancies often deliver earlier than 40 weeks, and providers plan their care differently.

Worked examples

Example 1: LMP January 1, 2026, 28-day cycle

January 1 + 280 days = October 8, 2026. Check: +1 year − 3 months + 7 days → October 8.

Example 2: Same LMP, 32-day cycle

Cycle is 4 days longer than 28, so add 4 days: October 12, 2026.

Limitations

• Assumes you know your LMP date and that cycles are regular.
• Not accurate for irregular cycles, recent hormonal contraception, or pregnancies after IVF.
• Ultrasound dating in early pregnancy is more accurate.

Who should not rely on this calculator

This tool cannot confirm a pregnancy or replace prenatal care. If you think you are pregnant, contact a healthcare provider to start prenatal care. Seek urgent care for bleeding, severe pain, or other concerning symptoms.