How to Calculate Your Due Date

What is the due date formula?

The standard formula is Naegele's rule: take the first day of your last menstrual period (LMP) and add 280 days. Your estimated due date (EDD) is the day 40 weeks later. A shortcut that gives the same answer is to add one year, subtract three months and add seven days.

Pregnancy is counted from the last period, not from conception, because the period date is usually known and ovulation isn't. That is why you are already considered "2 weeks pregnant" around the time you conceive. Our due date calculator uses this same convention for every method.

• EDD = LMP + 280 days
• Naegele's shortcut: LMP + 1 year − 3 months + 7 days
• Cycle adjustment: add (cycle length − 28) days
• From conception: conception date + 266 days
• From IVF: transfer date + 266 days − embryo age at transfer

How do I use Naegele's rule? (worked example)

Say your last period started on Thursday, January 1, 2026. Add 7 days to get January 8, 2026, subtract 3 months to get October 8, 2025, then add a year. Your estimated due date is Thursday, October 8, 2026. Counting 280 days forward from January 1 lands on the same day.

The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is regularly longer or shorter, shift the date by the difference. With a 32-day cycle, ovulation probably happened about 4 days later, so the due date moves to Monday, October 12, 2026. With a 25-day cycle, it moves 3 days earlier, to Monday, October 5, 2026. Our calculator makes this adjustment when you choose your cycle length.

How do I calculate my due date from conception or IVF?

If you know the date you conceived, add 266 days (38 weeks). That is 280 days minus the roughly 14 days between the start of a period and ovulation. A conception date of January 15, 2026 gives a due date of October 8, 2026, the same as the LMP example above.

IVF gives the most precise timing because the embryo's age is known. The American College of Obstetricians and Gynecologists (ACOG) dates IVF pregnancies from the transfer date and the embryo's age: add 266 days, then subtract the number of days the embryo had developed. For a day-5 blastocyst that means transfer date + 261 days, and for a day-3 embryo, + 263 days.

How does ultrasound change the due date?

An ultrasound in the first trimester is the most accurate way to estimate gestational age, according to ACOG, and it can replace the date from your last period. Up to 13 weeks and 6 days, the measurement used is crown-rump length, the length of the embryo or fetus from head to bottom.

ACOG does not change the date for every small difference. It keeps the LMP-based date unless the ultrasound estimate differs by more than a set number of days, and that allowance grows later in pregnancy because babies' sizes vary more as they grow.

Example: your period-based due date is October 8, 2026, and an ultrasound on March 12, 2026 measures the pregnancy at 9 weeks 4 days. Counting back from the scan gives a due date of October 11, 2026. The difference is 3 days, less than the 7-day allowance at that stage, so the October 8 date would normally stand. If you only have an ultrasound date, the calculator's ultrasound option works out the due date from the scan date and the gestational age on the report.

When do the trimesters start and end?

Counted from the first day of the last period, the first trimester runs through 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth. For the January 1, 2026 example, our calculator gives these dates. The pregnancy calculator shows how far along you are on any given day.

How accurate is a due date?

A due date is an estimate, not a deadline. Only about 4% of babies are born on the exact date, and about 70% arrive within 10 days of it. In a small study of 125 naturally conceived pregnancies with known ovulation dates, pregnancy length still varied by 37 days from shortest to longest. ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days, a two-week span rather than one day.

Three things make the estimate less certain: an irregular cycle, not remembering the date of the last period, and recent hormonal birth control or breastfeeding, which can delay ovulation. In those cases, an early ultrasound is especially useful. Your doctor or midwife will settle on one official due date and use it for the rest of the pregnancy.

Can I work out when I conceived from my due date?

Yes, roughly: subtract 266 days from the due date to get the most likely conception date. For a due date of October 8, 2026, that is January 15, 2026. Because sperm can survive several days and ovulation timing varies, the pregnancy conception calculator gives a window, here January 12 to 18, 2026, rather than a single day.

If you are not pregnant yet and want to plan, the conception calculator projects fertile days and the due date that would follow from conceiving this cycle. Our guide to when you are ovulating explains how those fertile days are estimated.