Pregnancy Due Date Calculator

How Pregnancy Due Dates Are Estimated

A pregnancy due date calculator gives you a single date — your estimated due date, or EDD — but the real value of running the calculation early is that everything else in your NHS antenatal pathway lines up against it. Booking visit, dating scan, anomaly scan, glucose tolerance test, whooping cough vaccine, anti-D injection, MATB1 form and your statutory maternity leave start date all key off the EDD. Get the date right and the rest of the schedule writes itself.

NHS antenatal care in England, Scotland, Wales and Northern Ireland follows the NICE guideline NG201 — the modern updated antenatal pathway that replaced earlier NICE guidance. NG201 sets out a series of contacts between you and your midwife from booking through to birth. First-time mothers (nulliparous) are offered a schedule of around ten contacts; women who have given birth before (multiparous) and have an uncomplicated pregnancy are offered around seven. The first of these is the booking visit, and it should happen by week 10 of pregnancy — ideally before week 12 weeks and 6 days — so that your dating scan can be booked into the right window.

The key fixed points in the NHS pathway are the booking appointment by week 10, the dating scan between 11 weeks 0 days and 13 weeks 6 days, the anomaly scan between 18 weeks 0 days and 20 weeks 6 days, the glucose tolerance test at 24–28 weeks for those at risk of gestational diabetes, the whooping cough (pertussis) vaccine between 16 and 32 weeks, the seasonal flu jab at any point during flu season, and — for women who are Rhesus negative — the anti-D immunoglobulin injection at around 28 weeks. From 20 weeks your midwife can issue your MATB1 form, the certificate you give to your employer to claim Statutory Maternity Pay.

This page calculates your EDD using Naegele's Rule, then lays the full NHS schedule on top of it so you can see the booking visit window, both scan windows, vaccine windows and the MATB1 and SMP qualifying-week dates that govern your maternity leave. Always confirm the calculated dates with your midwife or GP — the dating scan in particular can shift your EDD by up to five days in either direction, which then shifts every appointment that hangs off it. Around 600,000 babies are born in England and Wales each year and only about 5% arrive on the exact EDD, so treat the date as a planning anchor, not a deadline.

How to Calculate Your Due Date From Your Last Period

Enter the first day of your last menstrual period, your average cycle length, or — if you conceived through IVF — your embryo transfer date and embryo age. The calculator returns your estimated due date (EDD), current gestational age in weeks and days, your trimester, and the dates of your NHS booking visit, dating scan, anomaly scan and MATB1 window.

1. Choose the calculation method that matches your situation: LMP (last menstrual period), conception/ovulation date, or IVF embryo transfer.
2. If using LMP, enter the first day of your most recent period — not the last day, and not the day you got a positive test.
3. If your cycle is not 28 days, enter your average cycle length so the calculator can adjust the ovulation assumption.
4. If using IVF, enter the embryo transfer date and select whether the embryo was a 3-day or 5-day blastocyst.
5. Click Calculate to generate your EDD, current gestational age in weeks and days, and trimester.
6. Review the antenatal milestone table for the dates of your booking visit, dating scan, anomaly scan, GTT, vaccine windows, MATB1 issue date and SMP qualifying week.
7. Bring the printed result to your booking appointment so the midwife can cross-check against the dating scan.

Naegele's Rule: The Standard Due Date Formula

The formula used: EDD = LMP + 280 days (Naegele's Rule)

Due Date Examples for Different LMP Dates

First baby, regular 28-day cycle, LMP 15 March 2026 → EDD 22 December 2026. Booking visit by 10 April. Dating scan window 1–22 June. Anomaly scan window 20 July–10 August. MATB1 issued from 3 August. SMP qualifying week begins 6 September.

When to Use a Due Date Calculator (Before Your First Scan)

Run this calculator the day you get a positive home pregnancy test. The first NHS task — booking the midwife appointment via your GP surgery or local maternity self-referral website — should happen by week 10 of pregnancy, and most maternity units in England and Scotland are now booked four to six weeks in advance, so leaving it until you have your first symptoms can put the appointment outside the recommended window.

The second reason to run the calculator early is the dating scan. The 11+0 to 13+6 weeks window is narrow, only three weeks wide, and it is the same window used for the combined screening test (nuchal translucency plus the PAPP-A and beta-hCG blood tests) which informs Down's, Edwards' and Patau's syndrome risk assessment. Miss the window and you cannot have combined screening — only the quadruple blood test offered between 14+2 and 20+0 weeks remains, which screens for Down's only and is less sensitive.

The third reason is maternity leave planning. Statutory Maternity Leave can start any time from 11 weeks before the expected week of childbirth (EWC). The earliest start date and your SMP qualifying week — the 15th week before the EWC, which is the cut-off for proving you've worked for your employer for 26 continuous weeks — both depend on the EDD. If you intend to give your employer the standard 28 days' notice of leave start date and the MATB1 form, the calculator's projected MATB1 issue date (from 20 weeks) is the date you can begin that paperwork.

LMP, Gestational Age, and Trimester Definitions

Due Date Methods: LMP vs Ultrasound vs Conception Date

Tips for Tracking Pregnancy Milestones

• Book your first midwife appointment via your GP surgery or local NHS self-referral page within a week of a positive pregnancy test — most areas are running 4–6 week waits.
• Start 400 micrograms of folic acid daily as soon as you suspect pregnancy (ideally from the moment you start trying) and continue until the end of week 12. Take 10 micrograms of vitamin D daily throughout pregnancy.
• Save the dating-scan window dates (11+0 to 13+6 weeks) into your calendar — the combined screening test depends on it.
• Ask your midwife at the booking visit for the Group B Streptococcus information leaflet and the seasonal flu and pertussis vaccine plan.
• If you are Rhesus-negative, confirm your 28-week anti-D appointment is in the diary at the 25-week (first pregnancy) or 28-week midwife check.
• Request your MATB1 form from your midwife at the appointment closest to 20 weeks — you cannot claim Statutory Maternity Pay without it.
• Give your employer at least 28 days' notice of your maternity leave start date; the MATB1 must be submitted no later than the end of the 15th week before your EDD.

Due Date Estimation Errors Expectant Parents Make

• Entering the date of your positive pregnancy test instead of the first day of your last menstrual period — the LMP is what Naegele's Rule and every NHS form depend on.
• Confusing gestational age (counted from LMP) with fetal age (counted from conception). Fetal age is roughly two weeks less. NHS records always use gestational age.
• Treating the EDD as a deadline. Full term runs from 37 weeks 0 days to 41 weeks 6 days; only about 5% of babies arrive on the exact date.
• Missing the 11+0 to 13+6 dating-scan window because of late booking. Combined screening is only available inside it.
• Assuming an IVF transfer date is the same as the LMP. A 5-day blastocyst transfer is dated as if the LMP was 19 days earlier, not the transfer day itself.
• Forgetting to ask for the MATB1 at the 20-week appointment. Without it, your employer cannot process Statutory Maternity Pay and HMRC cannot start Maternity Allowance.