Due Date Calculator
Estimate your due date, current week of pregnancy, and ovulation window in one step.
Estimates use standard 280-day (40-week) dating from your last period, adjusted for cycle length, or 266 days from conception. Only about 5% of babies arrive exactly on their due date — an ultrasound scan gives a more precise estimate.
How your due date is calculated
The standard method for estimating a due date, known as Naegele's rule, counts 280 days — 40 weeks — from the first day of your last menstrual period. That's because pregnancy is conventionally measured from the start of your last period, not the actual date of conception, which typically happens about two weeks later. If your cycle is longer or shorter than the average 28 days, the estimate shifts slightly to reflect a later or earlier ovulation.
If you know your conception date instead — for example from fertility tracking or IVF — this calculator instead adds 266 days (38 weeks) directly from that date.
Understanding trimesters and pregnancy weeks
- First trimester: weeks 1 through 13
- Second trimester: weeks 14 through 27
- Third trimester: weeks 28 through 40
Frequently asked questions
How accurate is a due date calculator?
A due date estimate based on your last period is accurate within about one to two weeks for most pregnancies. Only about 5% of babies are born on their exact estimated due date. An ultrasound dating scan is more precise.
How is the due date calculated from the last period?
The most common method, called Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period, then adjusts for cycle length if it's longer or shorter than 28 days.
When am I most likely to ovulate?
Ovulation typically happens about 14 days before your next period starts, which for a 28-day cycle is around day 14.