How your due date is estimated
Pregnancy is counted from the first day of your last menstrual period (LMP), about two weeks before conception. The standard estimate, Naegele’s rule, places the due date 280 days, or 40 weeks, after that date. The calculator adjusts for your cycle length, and can also start from a known conception date or from an ultrasound measurement.
The methods
- Last period: due date = LMP + 280 days + (cycle length − 28 days).
- Conception date: due date = conception + 266 days (38 weeks).
- Ultrasound: due date = scan date − pregnancy age at the scan + 280 days.
An early ultrasound, between 8 and 14 weeks, measures the crown-rump length of the embryo and is the most accurate way to date a pregnancy. When it differs enough from the period date, your provider will use the ultrasound date.
Worked example
Your last period started on August 25, 2026 and your cycle is 28 days.
- Due date: August 25, 2026 + 280 days = Tuesday, June 1, 2027.
- Conception most likely happened around September 8, 2026.
- On November 3, 2026, you are 10 weeks and 0 days pregnant, in the first trimester, with 210 days to go.
With a 32-day cycle, ovulation comes 4 days later, and so does the due date: June 5, 2027.
Key dates
| Term | Weeks |
|---|---|
| Preterm | before 37 weeks 0 days |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Postterm | 42 weeks and beyond |
These definitions from ACOG explain why the calculator shows the start of full term (39 weeks) and 41 weeks alongside the due date.
How often babies arrive on time
Only about 4 to 5% of babies are born on their due date. About half arrive within a week of it and most within two weeks either side. First babies tend to come a little later than subsequent ones on average.
Planning around the date
Use the due date to plan appointments, prenatal tests and leave from work. Common milestones include the first-trimester screening around 11 to 14 weeks, the anatomy scan around 18 to 22 weeks and the glucose test around 24 to 28 weeks.
A note on medical advice
This calculator gives estimates based on averages. It does not replace advice from your doctor, midwife or health care provider, who will date your pregnancy and follow it with exams and ultrasounds.