How to use the due date calculator
Pick the first day of your last menstrual period (LMP) and enter your average cycle length – the number of days from the start of one period to the start of the next. The calculator gives your estimated due date, how many weeks and days pregnant you are today, the estimated date of conception and when each trimester begins. A milestone table lists dates for common prenatal checkpoints.
How the due date is calculated
Doctors and midwives traditionally use Naegele's rule: add 280 days (40 weeks) to the first day of the last period. This assumes a 28-day cycle in which ovulation occurs around day 14. If your cycle is longer or shorter, ovulation shifts too, so the calculator moves the due date by the difference:
Pregnancy is counted from the LMP, not from conception, which is why a "40-week" pregnancy actually lasts about 38 weeks from fertilization.
How accurate is a due date?
Only about 4–5% of babies arrive on their exact due date. Most are born within a week or two either side, and a birth between 37 and 42 weeks is considered term. An early ultrasound, ideally in the first trimester, measures the baby and is the most accurate way to date a pregnancy; your provider may adjust the date based on it.
The trimesters
- First trimester (weeks 1–13): major organs form; nausea and fatigue are common.
- Second trimester (weeks 14–27): many people feel better; the anatomy scan happens around week 20 and movements become noticeable.
- Third trimester (week 28 to birth): rapid growth and preparation for delivery.
This calculator is for information only and does not replace care from a qualified healthcare provider.
Worked example
If the first day of your last period was 1 March and your cycle is a regular 28 days, the estimated due date is 280 days later – 6 December. With a 32-day cycle, ovulation probably occurred about four days later than average, so the estimate moves to 10 December.
Frequently asked questions
How is my due date calculated?
By adding 280 days to the first day of your last period and adjusting for cycle lengths that differ from 28 days.
What if I don’t know the date of my last period?
An early ultrasound can estimate gestational age accurately. Ask your healthcare provider.
Can my due date change?
Yes. Providers often update it after the first ultrasound if the measurements differ noticeably from the date-based estimate.
Is it normal to deliver before or after the due date?
Yes. Most babies are born within two weeks of the due date, and birth between 37 and 42 weeks is considered term.
Last reviewed 2026-10-27. This due date calculator is provided for general information; verify important decisions with a qualified professional.