How do I calculate my due date?

The most common method is Naegele's Rule: take the first day of your last menstrual period (LMP), subtract 3 months, then add 7 days. Alternatively, add 280 days (40 weeks) to your LMP. Example: LMP = January 1 → due date = October 8.

How accurate is a due date calculation?

Only about 5% of babies are born on their exact due date. About 80% are born within 2 weeks either side. The due date marks the midpoint of a normal delivery window (38-42 weeks). First trimester ultrasound (between 8-13 weeks) gives the most accurate due date within plus or minus 5 days.

What does 40 weeks pregnant mean?

Pregnancy is measured in weeks from the first day of your last menstrual period (LMP), not from conception. At week 40, which is your due date, the baby is actually about 38 weeks old from conception. Full-term pregnancy is defined as 39-40 weeks; 37-38 weeks is early term; 41 weeks is late term; 42+ weeks is post-term.

How is a due date calculated after IVF?

With IVF, the conception date is known precisely. The due date is calculated as 266 days (38 weeks) from the date of fertilization for day-0 embryos, or 263 days (37 weeks 6 days) from egg retrieval for day-3 embryos, or 261 days for day-5 embryos (blastocyst transfer).

Pregnancy Due Date Calculator

Last menstrual period (LMP)

About the Pregnancy Due Date Calculator

This tool estimates expected due date (EDD) and gestational age using common clinical rules. Enter your Last Menstrual Period (LMP) date to get your estimated due date, trimester, and gestational age.

How to use

Enter the first day of your Last Menstrual Period (LMP) to compute EDD and gestational age.
Or provide a known conception date (or IVF date) for more precise dating.

Assumptions

Standard pregnancy length from LMP: 280 days (40 weeks).
Conception typically ~14 days after LMP.

Examples

If LMP = 2026-01-01 → EDD = 2026-10-08. On 2026-04-01 gestational age ≈ 13w 6d.
If conception known as 2026-02-01 → estimated due ≈ 2026-11-24 (conception + 266 days).

When to consult a provider

If you need precise dating (e.g. IVF, pregnancy complications), schedule an early ultrasound; it is more accurate than LMP-based estimates.

How Due Dates Are Calculated

The standard method (Naegele's rule) calculates due date as: LMP (last menstrual period) + 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14. From conception date, add 266 days (38 weeks). Ultrasound dating at 8–12 weeks gives a crown-rump length (CRL) that is used to estimate gestational age with ±5 days accuracy — often more reliable than LMP-based calculation when cycle length varies.

Only 5% of babies are born on their exact due date; 90% are born between 37 and 42 weeks. 'Full term' is 39–40 weeks; 'early term' 37–38 weeks; 'late term' 41 weeks; 'post-term' 42+ weeks. Your healthcare provider uses both the LMP date and ultrasound measurements to set a consistent estimated due date (EDD). This calculator provides an estimate for planning — your obstetric team will confirm and adjust as needed.

Milestone	Weeks from LMP	From Conception
Positive test typical	4–5 weeks	2–3 weeks
First trimester ends	13 weeks	11 weeks
Anatomy scan	18–22 weeks	16–20 weeks
Viability threshold	24 weeks	22 weeks
'Early term' birth	37 weeks	35 weeks
Full term birth	39–40 weeks	37–38 weeks

Milestone

Weeks from LMP

From Conception

Positive test typical

4–5 weeks

2–3 weeks

First trimester ends

13 weeks

11 weeks

Anatomy scan

18–22 weeks

16–20 weeks

Viability threshold

24 weeks

22 weeks

'Early term' birth

37 weeks

35 weeks

Full term birth

39–40 weeks

37–38 weeks