How is the due date calculated?

The standard method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). For cycles other than 28 days, ovulation occurs earlier or later, so the due date shifts by the same number of days. Ultrasound dating in the first trimester is more accurate and is typically used to confirm or adjust the LMP-based estimate.

How accurate is a due date calculator?

LMP-based due date calculators are accurate within about 5–7 days for women with regular 28-day cycles. For irregular cycles, accuracy decreases. Only about 5% of babies are born on their exact due date — most arrive within a 2-week window. First-trimester ultrasound (at 8–12 weeks) is the gold standard for dating accuracy.

What is gestational age?

Gestational age counts pregnancy from the first day of your last period, not from conception. This means gestational age at birth is typically 40 weeks, even though the embryo is only about 38 weeks old from fertilisation. Healthcare providers use gestational age universally because the exact date of conception is rarely known.

What if my cycle is irregular?

This calculator adjusts for cycle length by shifting ovulation timing. If your cycles vary widely (by more than 5–7 days cycle to cycle), the LMP method is less reliable. In this case, an early ultrasound is the best way to establish an accurate due date. Irregular cycles are common and your doctor can advise on how to track ovulation.

What does 'full term' mean?

A pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks; late term is 41 weeks; post-term is 42 weeks or beyond. Babies born before 37 weeks are considered premature (preterm). The weeks 37–42 are all within the normal range for delivery.

Can I use this calculator for IVF pregnancies?

For IVF pregnancies, due dates are calculated differently — typically 266 days (38 weeks) from the egg retrieval date, or adjusted based on the embryo transfer day (day 3 or day 5 transfer). This calculator uses LMP-based dating, which is not appropriate for IVF. Use the dates provided by your fertility clinic.

What are the signs of early pregnancy?

Early signs include missed period, nausea (morning sickness), breast tenderness, fatigue, frequent urination, and light spotting (implantation bleeding). A home pregnancy test can detect hCG hormone from around the time of a missed period. Always confirm with a healthcare provider for a clinical diagnosis.

When should I book my first prenatal appointment?

Most healthcare providers recommend booking your first prenatal appointment at 8–10 weeks of pregnancy. In the UK, the first midwife booking appointment is typically at 8–10 weeks with a dating scan at 11–14 weeks. In the US, the first OB visit is usually at 8 weeks.

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Pregnancy Due Date Calculator

Day of Last Period

Month of Last Period

Year of Last Period

yyyy

Average Cycle Length

days

About this tool

Estimate your due date and key milestones

The Pregnancy Due Date Calculator estimates your expected delivery date (EDD) based on the first day of your last menstrual period (LMP) and your average cycle length. It uses Naegele's Rule — the same formula used in clinical practice — adjusted for cycles that differ from the standard 28 days.

Naegele's Rule: add 280 days (40 weeks) to the first day of your LMP. For cycles longer or shorter than 28 days, ovulation shifts by the same number of days, so the formula adjusts accordingly.

1st trimester (weeks 1–13) — organ formation, highest risk period
2nd trimester (weeks 14–27) — typically the most comfortable stage
3rd trimester (weeks 28–40+) — baby gains weight, lungs mature

Important: This calculator is for informational purposes only. Only your healthcare provider can confirm your due date, typically through ultrasound dating in the first trimester, which is more accurate than LMP-based calculations.

Example

LMP: 1 March 2025, cycle length: 28 days.

Due date = 1 March + 280 days = 5 December 2025.

For a 35-day cycle, ovulation shifts +7 days, so due date = 1 March + 287 days = 12 December 2025.

Only about 5% of babies are born on their exact due date — most arrive within 2 weeks either side.