🤰 How to Calculate Your Pregnancy Due Date From Your Last Period

The positive test tells you that you are expecting — the next question is always when. A pregnancy due date (the estimated date of delivery, or EDD) is the anchor for every prenatal appointment, every trimester milestone, and every plan you make around the baby's arrival. You don't need a doctor's visit to get a good estimate: the standard medical method, Naegele's rule, needs only one date — the first day of your last menstrual period (LMP). Here is the exact formula, a worked example, and how to adjust it for your own cycle.

Naegele's rule: the standard formula

Doctors have used the same method for over two centuries. Naegele's rule, named after German obstetrician Franz Naegele, takes the first day of your last menstrual period and adds 280 days — exactly 40 weeks. That date is your estimated due date.

There is also a shortcut version that gives the same answer without counting 280 days: take the first day of your last period, add 7 days, subtract 3 months, and add 1 year.

The rule quietly assumes two things: a regular 28-day cycle, and ovulation on day 14. If your cycles run like clockwork at 28 days, the formula is a solid estimate. If they don't — and for many women they don't — you need the small adjustment below.

Worked example

Say the first day of your last period was 5 January 2026. Using the shortcut: 5 January + 7 days = 12 January; minus 3 months = 12 October 2025; plus 1 year = 12 October 2026. Estimated due date: 12 October 2026.

As a check, counting 280 days straight from 5 January 2026 lands on exactly the same day — 12 October 2026. Run your own dates in our free Pregnancy Due Date Calculator — it applies the same rule and also tells you your current week and trimester.

What if your cycles are longer or shorter than 28 days?

Here is why a simple adjustment works: the second half of the cycle (after ovulation, the luteal phase) is remarkably constant at about 14 days for most people — almost all cycle-length variation comes from the first half. So if your cycle is a regular 35 days, you probably ovulated around day 21 instead of day 14, roughly 7 days later than the rule assumes.

The standard adjustment: add one day to the due date for each day your cycle is longer than 28, and subtract one day for each day it is shorter. With the 5 January 2026 LMP and a 35-day cycle, the estimated due date moves from 12 October to 19 October 2026. For irregular cycles there is no reliable adjustment — let your doctor know the dates were irregular, because an early ultrasound is a far better anchor in that case.

Why is pregnancy counted as 40 weeks when conception is at week 2?

Because your last-period date is usually known exactly, while the conception date almost never is. At the moment of conception the pregnancy is already counted as about 2 weeks along — gestational age runs about two weeks ahead of the embryo's true age. The full 40 weeks therefore include those two pre-conception weeks, with birth expected roughly 38 weeks after ovulation. It is the dating convention every provider uses, so sticking with it keeps you on the same page as your care team.

How accurate is a pregnancy due date?

Think of it as the middle of a window, not a deadline. Only about 4–5% of babies are born on their estimated due date. Birth anywhere between 37 and 42 weeks is considered term, and most babies arrive within a couple of weeks either side of the date. If the single date is stressing you out, think in terms of a "due month" — the four weeks centred on your EDD — which captures the large majority of births.

Is an ultrasound due date more accurate?

A first-trimester ultrasound (up to 13 weeks 6 days) is the most accurate way to date a pregnancy, because early embryonic growth follows a very predictable pattern. Doctors switch to the ultrasound date when it differs from the LMP-based date by more than 5 days before 9 weeks, or by more than 7 days between 9 weeks and 15 weeks 6 days. Later scans are less reliable for dating, so if you had an early scan, that date is the one to trust. For anything that affects your care, always confirm the timing with your healthcare provider — these tools inform, they do not replace medical guidance.

📚 Related guide: Once you know the date, read our How Much Sleep Do You Need by Age? guide — handy for the newborn months — and use our Sleep Calculator to work out your bedtime.