Pregnancy Due Date Calculator
This calculator estimates your due date from the first day of your last menstrual period (the default method in maternity care) or from a known conception or ovulation date. It also shows how far along you are, when each trimester begins, and when full term starts.
Pregnancy is counted from the last period, not conception, so a "40-week pregnancy" includes about two weeks before conception. The calculator handles that convention for you.
How due dates are estimated
From last period: Due date = LMP + 280 days, adjusted by (cycle length − 28)
From conception: Due date = Conception date + 266 days
The 280-day rule (Naegele's rule) assumes ovulation on day 14 of a 28-day cycle. If your cycles run longer or shorter, ovulation likely shifted too, so the calculator shifts the due date by the same number of days. A 32-day cycle pushes the estimate 4 days later. Trimesters follow the common clinical convention: the second begins at week 14 and the third at week 28.
Worked example
Last period started March 10, with a typical 30-day cycle:
| Standard 280 days | March 10 + 280 days = December 15 |
|---|---|
| Cycle adjustment | +2 days (30 − 28), giving December 17 |
| On June 10 you'd be | 12 weeks 6 days, the day before the second trimester |
Only about 4–5% of babies arrive on their exact due date. Most arrive within two weeks either side. Think of the date as the center of a window, not an appointment.
What happens to this date next
Your midwife or doctor will refine the estimate at the dating ultrasound (usually between 8 and 14 weeks), which measures the embryo directly and is more accurate than any calendar method. If the scan disagrees with the period-based date by more than a few days, care teams generally adopt the scan date. The week count then drives the schedule of screening tests, the anomaly scan around week 20, glucose testing, and the term milestones (early term from 37 weeks, full term from 39).
If your cycles are irregular or you are unsure of your dates, do not worry about the calculator's precision. That is what the dating scan is for. Bring whatever dates you have to your first appointment.
Frequently asked questions
How accurate is a due date calculator?
Calendar methods are accurate to within a few days if your cycle data is right, but only about 1 in 20 babies arrives on the exact date. A first-trimester ultrasound is the most accurate dating method and will confirm or adjust this estimate.
How many weeks pregnant am I?
Enter your last period date above and the result shows your gestational age in weeks and days. Counting starts from the last period, so you are "4 weeks pregnant" around the time a period is first missed.
What if I don't know my last period date?
Use the conception-date option if you know when you ovulated or conceived, for example from tracking or fertility treatment. If neither date is known, your care provider will date the pregnancy by ultrasound, which is routine and reliable.
When do the trimesters start and end?
By the convention used here: first trimester from week 1 through 13, second from week 14 through 27, third from week 28 to birth. Definitions vary slightly between sources, so an app may draw the lines a week differently.
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This calculator provides an estimate for general information only and is not medical advice. Confirm your dates with your midwife, doctor or an ultrasound scan, and seek professional care for any concerns during pregnancy.