How to Use Our Pregnancy Due Date Calculator

Our pregnancy due date calculator uses Naegele's rule — the standard formula used by the NHS and NICE for estimating when your baby will arrive. This guide walks you through entering your information, understanding the results, and using your due date to plan ahead.

Why Use a Pregnancy Due Date Calculator

When you use a pregnancy due date calculator, you get an evidence-based estimate of when to expect your baby, which weeks of pregnancy you're currently in, and key trimester dates. Naegele's rule — counting forward 40 weeks from the first day of your last menstrual period (LMP) — is the method recommended by the NHS pregnancy due date guidance and NICE antenatal care guideline NG201.

A calculator does the maths for you. It also shows you your current week of pregnancy and flags key milestones (end of first trimester, viability point, full term) so you can sync them with your plans and appointments. You'll have your due date in under a minute — no calculator needed, but good to have the certainty.

What You'll Need Before Starting

To use the calculator accurately, gather one of these dates:

First day of your last menstrual period (LMP) — this is the most reliable starting point, and is what most hospitals use. If you track your cycle or have this in a calendar or period app, grab it now.
Your conception date — if you know when conception likely happened (helpful if you're tracking ovulation or had fertility treatment), the calculator can work backwards. This is less precise than LMP but still useful.
Your dating scan result — if you've already had a dating ultrasound at 8–14 weeks, that's the most accurate data. You can use the scan date and your gestational age at that scan.

If your cycle is irregular, you're not sure of your dates, or you've used fertility treatment, don't worry — the calculator will still give you a reasonable estimate, and your midwife or consultant will refine it at your dating scan. Having any of these three pieces of information is enough to get started.

Step-by-Step: How to Calculate Your Due Date

Step 1: Choose your input method The calculator offers three options: enter your LMP date, your conception date, or your dating scan result. Pick the one you know most confidently. The fields are pre-filled with today's date as a default, but change them to match your dates.

Step 2: Enter your date Click or tap the date field and choose the relevant date. The calculator will immediately show you:

Your estimated due date
Your current week and day of pregnancy
The dates when you'll reach key milestones (end of first trimester, viability, full term)

If you're unsure of an exact date (e.g., "I think it was the third week of March"), enter your best estimate — the calculator will give you a range of ±5 days anyway, so precision to the exact day matters less than you might think.

Step 3: Review the milestones The calculator shows you:

Due date — 40 weeks from LMP (or adjusted if you entered conception or scan date)
Current pregnancy week — how far along you are right now
First trimester ends — week 12 (often the "public announcement" moment)
Viability point — week 23 (the point from which survival rates outside the womb become significant, though outcomes are much better from week 28 onwards)
Full term — week 37 onwards (technically any time after this is term, though 39–40 weeks is optimal for most pregnancies)

These milestones help you align your decisions — when to tell work, when to book key scans, when to prepare the nursery.

Step 4: Use the countdown Knowing how many weeks until your due date is helpful for planning. You can also use our date countdown timer to create a specific countdown if you want to track the days, hours, or minutes. (Some people find this motivating; others find it anxiety-inducing — suit yourself.)

Step 5: Check back as you progress Each time you have a dating scan or an appointment, your estimated due date might adjust slightly. You can re-enter your updated information into the calculator to track how the estimate evolves. If a midwife gives you a new EDD at your dating scan, use that — it's based on the most accurate information. If you're planning a pregnancy, our fertility calculator can help you understand your cycle and conception window.

Understanding Your Results

Your due date is an estimate, not a deadline. Naegele's rule assumes a 28-day cycle and conception on day 14. If your cycle is longer or shorter, conception might have happened at a different point, which shifts your due date. Even if everything is perfectly timed, babies arrive when they're ready — only about 5% of babies arrive exactly on their due date. The NHS recognises "term" as 37–42 weeks; about 80% of pregnancies will deliver between 38 and 41 weeks.

Your current week of pregnancy is calculated from your LMP. This is the convention used by the NHS and midwives, even though you weren't actually pregnant for those first two weeks. At your dating scan (8–14 weeks), the sonographer measures your baby's size and may adjust your due date by a few days if the measurement is significantly different from expected.

Trimester dates:

First trimester ends at week 13 (this is often the milestone where people feel comfortable telling their employer or wider circle, though the choice is entirely personal).
Viability at week 23 is when a baby could theoretically survive outside the womb with intensive care. Many people find this date emotionally significant.
Full term at week 37+ means your baby is considered fully developed and could arrive safely anytime.

The calculator shows all of these, so you can sync your planning with your pregnancy milestones rather than guessing. Our date difference calculator is also useful if you want to calculate how old your baby will be at any future date — helpful for planning when to return to work or time off parental leave.

Tips for Using Your Due Date Calculator

Use exact dates where you have them — if you know your LMP to the exact day, enter it exactly. If you're estimating, the calculator builds in a margin anyway (±5 days is normal).
Cross-check with your dating scan — the most accurate due date estimate comes from an ultrasound at 8–14 weeks. If your scan date differs from your LMP-based estimate, trust the scan.
Plan around your due date, not to it — you're likely to deliver within 2–3 weeks of the calculated date. If you're due 15 May, pack your hospital bag by early May and keep flexibility in your plans through late May.
Track your pregnancy week for your own reference — knowing you're at "week 24" or "week 32" helps you understand what's happening in your body and which information (books, websites, midwife advice) is most relevant to you right now.
Share with your healthcare team — if you've calculated a due date yourself, mention it at your first appointment so your midwife or doctor can confirm it against their records and adjust if needed.

Frequently Asked Questions

What's the difference between my due date and my conception date? Your due date is 40 weeks from the first day of your last menstrual period (LMP). Conception typically happens 2 weeks after LMP, so your due date is roughly 38 weeks after conception. The NHS counts pregnancy from LMP even though you weren't technically pregnant then — it's just the convention that makes dating consistent across different cycles.

Can the calculator tell me the exact day my baby will arrive? No — and no calculator, app, or midwife can predict that with certainty. The due date is an estimate based on averages. Babies arrive within a range of about 38–41 weeks most of the time. The calculator gives you a central estimate, but add a margin of several days in either direction for real-world planning.

What if my cycle is irregular? If your cycle is longer or shorter than 28 days, your due date estimate is less reliable based on LMP alone. This is why the NHS recommends a dating scan at 8–14 weeks — it gives the sonographer a chance to measure your baby and adjust the due date if needed. Use the calculator as a rough estimate and plan to refine it at your scan.

What if I used fertility treatment? If you had IVF or other assisted conception, your due date can be calculated from your egg collection or embryo transfer date. These are very precise starting points. Enter the relevant date into the calculator (or your clinic will have given you an estimated due date already). A dating scan will confirm it.

Why do people say "due date" is misleading? Because it suggests a specific date when actually it's a rough estimate. The term "estimated date of delivery" (EDD) is more accurate, but "due date" is more commonly used. The calculator shows both, and the NHS guidance emphasises that term is 37–42 weeks — your baby doesn't have a strict deadline.

Can I use this calculator if I don't know my LMP? You can enter your best estimate, but a dating scan (available free on the NHS, usually at 8–14 weeks) is more reliable if you're unsure. If you're not yet pregnant but planning conception, our fertility calculator can help you identify likely ovulation dates and conception windows.

What's Naegele's rule and why do hospitals use it? It's the formula: due date = LMP + 40 weeks (or LMP + 280 days). It assumes a 28-day cycle and ovulation on day 14, giving an average pregnancy length of 266 days from conception. It's consistent, easy to apply, and works well for population-level estimates. Individual pregnancies vary, which is why scans refine it.

Is my baby in danger if I go past my due date? No — going to 41 or even 42 weeks is not unusual and is generally safe if monitored. The NHS offers induction at 42 weeks as a precaution because risks of stillbirth and other complications increase slightly after that point, but many pregnancies that go past the due date deliver healthy babies without intervention. Your midwife will discuss monitoring and options if you reach 41–42 weeks.