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Pregnancy Due Date Calculator

One of the first questions after a positive pregnancy test is also the most exciting: when is the baby due? This free pregnancy due date calculator gives you that estimate in seconds. Enter the first day of your last menstrual period and it adds 280 days — the standard 40 weeks of pregnancy — to work out your estimated due date. It also shows how many weeks and days pregnant you are today, which trimester you are in, and the estimated date of conception. Knowing your due date helps you plan prenatal appointments, understand which tests and scans are coming up, and work out practical things such as parental leave and nursery plans. Keep in mind that the date is an estimate, not an appointment: only a small share of babies arrive on their exact due date, and an early ultrasound scan usually gives a more accurate date than a calculation from your last period, especially if your cycles are irregular. Use this result as a helpful guide, and confirm it with your doctor or midwife at your first prenatal visit.

How to Use

1. Enter the first day of your last menstrual period — the first day of bleeding, not the last.
2. Enter your average cycle length if it is not 28 days; leave it at 28 if you are unsure.
3. Click Calculate to see your estimated due date, current week, trimester and conception date.
4. Take the result to your first prenatal appointment, where a scan can confirm or adjust the date.

Worked Example

If the first day of your last menstrual period was 1 January 2026, add 280 days to get the estimated due date: 1 January 2026 plus 280 days is 8 October 2026. That is 40 weeks of pregnancy, counted from the last period rather than from conception, because ovulation usually happens about two weeks after the period starts. With a regular 28-day cycle, conception is estimated at 1 January plus 14 days, which is 15 January 2026. On 1 April 2026, this pregnancy would be 12 weeks and 6 days along — in the first trimester, which runs from week 1 to week 13 — with 27 weeks and 1 day still to go until the due date. The second trimester would begin at week 14, around 2 April 2026, and the third at week 28, around 9 July 2026. Remember that this is an estimate: most babies arrive within two weeks either side of the due date, and a dating scan in the first trimester is more accurate than this calculation, especially if cycles are irregular or the last period date is uncertain.

Why Due Dates Are Only Estimates

The 280-day rule, known as Naegele's rule, assumes a regular 28-day cycle with ovulation on day 14, and that the pregnancy lasts exactly 40 weeks. In real life, cycles vary, ovulation can happen earlier or later, and healthy pregnancies commonly last anywhere from 37 to 42 weeks. That is why only about 5 percent of babies are born on their due date, and why doctors usually describe it as an estimated due date rather than a deadline. An ultrasound in the first trimester, measuring the baby from crown to rump, can date a pregnancy to within a few days and is the most reliable method when the last period is unknown or cycles are irregular. Later scans are less precise for dating, because babies grow at different rates. If your scan date differs from your period-based date by more than a few days, your doctor will usually use the scan date instead. Whatever date you use, it is best treated as the middle of a window: pack a hospital bag a few weeks early, keep your doctor's number close, and know the signs of labour from week 37 onward.