🤰 Due Date Calculator

How it works

A due date calculator estimates when a baby is likely to arrive, counting from the first day of the last menstrual period (LMP) rather than from conception. That sounds odd, but conception usually happens about two weeks after the LMP, and the LMP is a date people can actually remember — so medicine standardized on it. A full-term pregnancy is dated at 40 weeks from the LMP.

The estimate assumes a 28-day cycle with ovulation on day 14. Real cycles vary, and only about 4% of babies arrive on their exact due date — most come within two weeks either side — but the date is still the anchor for scheduling ultrasounds, screening tests, and checkups throughout the pregnancy.

The formula

The classic method is Naegele's rule, named after the German obstetrician Franz Karl Naegele who described it in the early 1800s: due date = LMP + 9 months + 7 days. Equivalently: subtract 3 months from the LMP, add 7 days, and add 1 year. Both routes land on the same date.

Worked example

If the first day of the last period was January 10, 2026: add 9 months to get October 10, 2026, then add 7 days to get October 17, 2026. Using the alternate route: January 10 minus 3 months is October 10, 2025; plus 7 days is October 17, 2025; plus 1 year is October 17, 2026. Same answer either way.

Frequently asked questions

My cycles are longer (or shorter) than 28 days. Is the date wrong?
Possibly, by a few days. The rule assumes ovulation on day 14, so a 35-day cycle shifts the real due date about a week later. An early dating ultrasound gives a more accurate estimate when cycles are irregular.

What if the pregnancy came from IVF?
Then the dates are actually more precise: clinics date the pregnancy from the egg retrieval or embryo transfer date rather than the LMP, so Naegele's rule is not needed.

Why does the exact date matter if babies rarely arrive on it?
Because the whole care schedule keys off it — when screening tests are offered, when growth is measured, and when doctors start watching closely for an overdue pregnancy (past 41–42 weeks).

How to read your due date

The estimate uses Naegele's rule, which counts 280 days, or 40 weeks, from the first day of your last menstrual period. The rule quietly assumes a 28 day cycle with ovulation on day 14, and real cycles are often shorter or longer. If your cycle runs 32 days, ovulation probably happened around day 18 and the true due date sits about 4 days later than the standard estimate; a 24 day cycle pulls it about 4 days earlier. This is why first trimester ultrasound, which sizes the embryo directly, is considered the most accurate dating method, and why your provider may revise the estimate after that scan, typically when it disagrees with the period based date by more than about 5 to 7 days. Hold the date loosely either way: only about 1 in 20 babies arrive on the exact day, most arrive within roughly two weeks on either side, and a pregnancy is considered full term anywhere from 37 to 42 weeks. If you know the conception date, for example from fertility treatment or precise tracking, adding 266 days, or 38 weeks, is more accurate than the period method. Personal questions about your pregnancy belong with your midwife or doctor, who can see the full picture.

A worked example

Suppose the first day of the last period was March 10, 2026, and the cycle is a regular 28 days. Naegele's rule adds 7 days, taking you to March 17, subtracts 3 months, taking you to December 17, and adds a year, giving an estimated due date of December 17, 2026. Conception most likely occurred around March 24, two weeks after the period began. Working the calendar forward, a dating scan on June 16, 2026 would find the pregnancy at exactly 14 weeks, since June 16 falls 98 days after March 10. If that scan measured the embryo as 13 weeks instead, the provider would likely move the due date about a week later, to late December, and the scan based date would be the one used from then on.

Common mistakes

• Entering the wrong bleeding episode. Implantation spotting or a light, short bleed that was not a true period can shift the estimate by several weeks if it is used as the start date.
• Assuming the due date is a deadline. It is the centre of a normal window of several weeks, and most babies choose some other day in it.
• Forgetting to mention irregular or long cycles at the first appointment. The standard estimate can be a week or more off, and the care team would rather know early.
• Being surprised when the date changes. A revision after the first trimester scan is routine practice and improves accuracy; it is not a sign that anything is wrong.
• Doing the date maths by hand across February in a leap year. Month lengths are exactly where manual Naegele calculations go wrong, which is what the calculator is for.

Frequently asked questions

How accurate is my due date?

Treat it as the centre of a window, not an appointment. Only about 1 in 20 babies arrive on the exact estimated day, while the large majority arrive within about two weeks either side. A first trimester ultrasound gives the most accurate dating and may shift the estimate by days.

What if I know the exact conception date?

Then use it: pregnancy runs about 266 days, or 38 weeks, from conception. Add 266 days to the conception date for an estimate that is usually closer than the period based one. This applies with fertility treatment, ovulation tracking, or one clearly known candidate date.

My cycle is 35 days. Does that change the date?

Yes. The standard estimate assumes ovulation on day 14 of a 28 day cycle. With a 35 day cycle you likely ovulated around day 21, so your due date falls about a week later than the calculator suggests. Mention your cycle length at your first appointment.

Can my due date change as the pregnancy goes on?

It can, and early on that is normal. If a first trimester dating scan disagrees with the period based estimate by more than about 5 to 7 days, most providers adopt the scan date. After the first trimester, dates are rarely changed, because babies then grow at individual rates.