Due Date Calculator

Enter the first day of your last period (or a conception or IVF transfer date) to get your estimated due date, your current week of pregnancy, and the dates of key milestones like the 20-week anatomy scan.

Put this calculator on your website — free

Copy one snippet and give your visitors a working Due Date Calculator.

How due dates are estimated

Pregnancy is conventionally counted as 280 days — 40 weeks — from the first day of the last menstrual period, not from conception. That sounds odd (it includes about two weeks before conception even happened), but it exists for a practical reason: most people know when their last period started, and almost nobody knows exactly when they ovulated. If your cycles run longer or shorter than 28 days, ovulation probably shifted with them, so the calculator moves the estimate by one day per day of cycle-length difference.

When the conception date is known, the count is 266 days from that day. And IVF removes the guesswork entirely: the embryo's age is known to the day, so a day-5 transfer adds 261 days and a day-3 transfer adds 263 — both arriving at the same 266 days from fertilization.

The formula

EDD = LMP + 280 days + (cycle length − 28)
EDD = conception date + 266 days
EDD = transfer date + 261 days (day-5 embryo)  or  + 263 days (day-3 embryo)

EDD is the estimated due date; LMP is the first day of the last menstrual period; cycle length is your average cycle in days. The cycle adjustment assumes a longer cycle means later ovulation — the luteal (post-ovulation) half of the cycle is fairly fixed, so extra days accumulate before ovulation, not after.

Worked example

Last period started January 10, 2026, 28-day cycles: count 280 days forward and you land on Saturday, October 17, 2026.

Same period date but 32-day cycles: the estimate shifts 4 days later, to Wednesday, October 21, 2026, because ovulation likely came around day 18 instead of day 14.

A due date is a bell curve, not a deadline

Only around 4–5% of babies arrive on their estimated due date. The realistic picture is a distribution centered near 40 weeks: most babies arrive somewhere in the two weeks either side, first babies tend to run slightly later on average, and anywhere from 37 to 42 weeks is within the range providers plan around. So treat the date this calculator gives you as the center of a window — useful for planning leave and packing a bag, not for scheduling anything to the day. One more thing worth knowing: a first-trimester ultrasound measures the embryo directly and dates pregnancy more accurately than period math, so if your early scan disagrees with this estimate by more than about a week, your provider will usually go with the scan. That's not a mistake — it's the estimate getting better.

Frequently asked questions

How accurate is a due date calculator?

It gives a well-grounded estimate, not a promise. Only about 4-5% of babies arrive on the due date itself; most arrive within two weeks either side of it. Think of the due date as the middle of a window, and expect your provider to fine-tune it after a first-trimester ultrasound.

Why does the calculator count from my last period instead of conception?

Because the first day of a period is a date most people actually know, while ovulation and conception usually aren't observed. By convention, pregnancy is dated at 280 days (40 weeks) from the last menstrual period — which means the count includes roughly two weeks before conception even happened.

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

Enter your average cycle length and the calculator shifts the estimate by one day for each day your cycle differs from 28. A 32-day cycle usually means ovulation came about four days later than the standard assumption, so the due date moves four days later too.

Why is an IVF due date calculated differently?

With IVF the embryo's exact age is known, so there's no guessing about ovulation. The convention is transfer date plus 261 days for a day-5 embryo (blastocyst) or plus 263 days for a day-3 embryo — both equivalent to 266 days from fertilization. IVF pregnancies have the most precisely dated due dates of all.

Can my due date change after an ultrasound?

Yes, and that's normal. A first-trimester ultrasound measures the embryo's crown-rump length, which tracks gestational age more reliably than period dates. If the scan estimate differs from the period-based date by more than about 5-7 days early on, providers typically re-date the pregnancy to match the scan.

Related calculators