Pregnancy Due Date Calculator Guide

Knowing your estimated due date (EDD) is one of the first things established after a positive pregnancy test. But how is it calculated, how accurate is it, and what does the number of "weeks pregnant" actually mean? This guide explains the maths behind due date calculation, what factors affect accuracy, and what the NHS scan dates mean for your pregnancy timeline.

The Standard Method: Naegele's Rule

The most widely used formula for calculating EDD from the first day of the last menstrual period (LMP) is Naegele's Rule, developed in the 19th century:

Example: LMP = 15 January 2025
EDD = 15 January + 280 days = 22 October 2025
Or: 15 January − 3 months = 15 October + 7 days = 22 October 2025 ✓

The 280-day figure assumes a 28-day menstrual cycle and ovulation occurring on day 14. For women with longer or shorter cycles, the EDD should be adjusted (see below).

Why Pregnancy Is Counted From LMP, Not Conception

This is a common point of confusion. Pregnancy is counted from the first day of the LMP — typically 2 weeks before conception. This seems counterintuitive, but it's done for practical reasons:

• The LMP date is known with certainty; the date of conception often isn't.
• The system standardises care and ensures everyone is on the same timeline.
• Historically, the LMP was the only reliable indicator available.

So "4 weeks pregnant" in obstetric terms means 4 weeks since LMP — typically only 2 weeks since actual fertilisation. A "40-week pregnancy" actually involves approximately 38 weeks of actual fetal development.

Adjusting for Cycle Length

Naegele's Rule assumes a 28-day cycle. If your cycle is different, adjust the EDD:

Example: 35-day cycle → add 7 days to standard EDD
Example: 21-day cycle → subtract 7 days from standard EDD

NHS Dating Scan and Why It May Change Your EDD

The NHS offers a dating scan (nuchal translucency scan) at 10–14 weeks, typically at 12 weeks. This ultrasound measures the crown-rump length (CRL) of the fetus, which gives a more accurate gestational age estimate than LMP alone, because:

• Many women have irregular cycles or are uncertain of their LMP date
• Implantation timing varies between individuals
• Early fetal development follows a predictable size-to-age relationship

If the scan date and LMP-based EDD differ by more than 5 days, the NHS uses the scan date. This is normal — it doesn't mean anything is wrong.

Pregnancy Weeks vs Calendar Months

Pregnancy is measured in weeks, not months — but people often ask "how many months pregnant?" Converting is slightly misleading because calendar months aren't equal:

How Accurate Is the EDD?

Only about 4% of babies are born on their exact EDD. The actual distribution of birth dates is:

• 37–42 weeks: "full term" — the normal range
• About 80% of babies arrive within 2 weeks of the EDD (±14 days)
• Before 37 weeks: preterm
• After 42 weeks: post-term (induction is usually offered)

First-time mothers tend to go a few days past their EDD on average. Second and subsequent pregnancies tend to be slightly shorter.

Important NHS Antenatal Appointments

Key milestones to plan around your EDD:

• Booking appointment: 8–10 weeks — initial midwife assessment, medical history, blood tests
• Dating scan: 10–14 weeks (ideally 12 weeks)
• Anomaly scan: 18–21 weeks — detailed check of fetal anatomy
• Glucose tolerance test: 24–28 weeks if risk factors for gestational diabetes
• Group B Strep test: 35–37 weeks (not routine NHS, but available privately)

Summary

EDD = LMP + 280 days (Naegele's Rule). Adjust for cycle length by adding or subtracting days from the 28-day baseline. The 12-week scan date will override the LMP-based EDD if they differ by more than 5 days. Only 4% of babies arrive exactly on their due date — the normal birth window is 37–42 weeks. Use our due date calculator for instant results and a trimester breakdown.