How is a due date calculated?

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 on day 14. If your cycle is longer or shorter, the date is adjusted. The NHS, ACOG (US), and most international obstetric guidelines use this 40-week standard.

Why do some countries count pregnancy differently?

Most countries count pregnancy as 40 weeks from LMP (last menstrual period), which means the first 2 weeks before conception are included. Some countries historically counted from conception (38 weeks), but the LMP-based 40-week count is now the international standard endorsed by WHO, NHS, ACOG, and RCOG. Ultrasound dating has largely replaced LMP dating for accuracy in the first trimester.

How accurate is an EDD?

Only about 5% of babies are born on their exact estimated due date. Around 80% are born within 2 weeks either side. A first trimester ultrasound (7–13 weeks) gives the most accurate dating — typically within 5–7 days. Later ultrasounds are less accurate. Term is defined as 37–42 weeks; full-term is 39–40 weeks.

How is IVF due date calculated?

For IVF pregnancies: Day 3 transfer — add 263 days (37 weeks + 6 days) to the transfer date, or equivalently subtract 17 days and add 280 days. Day 5 blastocyst transfer — add 261 days (37 weeks + 4 days), or subtract 19 days and add 280 days. IVF due dates are generally more accurate because fertilization date is precisely known.

🌍 Naegele's Rule (Global) 🇬🇧 NHS 40-Week 🇺🇸 ACOG Standard 🧬 IVF Transfer

Pregnancy Due Date Calculator

Calculate your Estimated Due Date (EDD) from your last menstrual period, conception date, or IVF transfer date. The 40-week (280-day) Naegele's rule is used globally by the NHS, ACOG, WHO, and all major obstetric bodies.

Quick Answer

EDD = LMP + 280 days (40 weeks). This is Naegele's rule, used by the NHS, ACOG (US), RANZCOG (Australia/NZ), and most obstetric guidelines globally. Only 5% of babies are born on the exact EDD — 80% are born within 2 weeks. Term is 37–42 weeks; full-term is 39–40 weeks. IVF due dates use the transfer date minus 17 days (3-day transfer) or 19 days (5-day blastocyst).

Calculation Method

First day of last period

Cycle length (days)

Average is 28 days. Adjust for your cycle (20–45 days).

Pregnancy Dating Guidelines by Country

Country / Authority	Method	Dating Scan
🇬🇧 UK (NHS)	LMP + 280 days; ultrasound confirms at 8–14 weeks	11–14 week dating scan offered to all
🇺🇸 USA (ACOG)	LMP + 280 days; ultrasound preferred if available	First trimester ultrasound recommended
🇨🇦 Canada (SOGC)	LMP + 280 days; ultrasound at 11–14 weeks	Government-funded first trimester scan
🇦🇺 Australia (RANZCOG)	LMP + 280 days; CRL measurement preferred	11–13+6 week nuchal translucency scan
🇩🇪 Germany	LMP + 280 days; ultrasound at weeks 9–12	3 state-funded ultrasounds during pregnancy
🇯🇵 Japan	LMP + 280 days; very high ultrasound usage (weekly in some clinics)	Frequent scans — Japan has highest ultrasound rate globally
🇮🇳 India (FOGSI)	LMP + 280 days; ultrasound dating	First trimester scan recommended in guidelines

Frequently Asked Questions

What is the difference between due date and full-term? ▾

Your due date (EDD) is at 40 weeks. "Full-term" is now defined as 39–40 weeks by ACOG and the NHS. "Early-term" is 37–38 weeks; "late-term" is 41 weeks; "post-term" is 42 weeks or beyond. Babies born at exactly 39–40 weeks have the best outcomes statistically.

Does the due date change if my cycle is not 28 days? ▾

Yes. If your cycle is longer than 28 days, ovulation occurs later, and your EDD should be pushed forward. If shorter, it's pushed back. For example, a 35-day cycle means ovulation around day 21 instead of day 14 — shift EDD forward by 7 days. This calculator adjusts for cycle length automatically. First-trimester ultrasound is more accurate than LMP dating for irregular cycles.

How accurate is the IVF due date calculator? ▾

IVF due dates are more accurate than LMP-based dates because the fertilization date is precisely known. Day 3 embryo transfer: EDD = transfer date + 263 days. Day 5 blastocyst transfer: EDD = transfer date + 261 days. These are equivalent to adding 280 days from the "LMP" calculated backward from the transfer date.

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Sources & Methodology

The estimated due date uses Naegele's rule (last menstrual period + 280 days); ultrasound dating may adjust this, per ACOG guidance.

Standards and figures reviewed 2026.