Pregnancy Due Date Calculator: Estimating Your Delivery Date
A due date is estimated by adding 280 days — 40 weeks — to the first day of your last menstrual period. That is Naegele's rule, the same method clinics use for an initial estimate, and it also gives you the current gestational age and which trimester you are in.
Why 40 weeks from your last period
Gestational age is counted from the last period rather than conception because ovulation timing is rarely known precisely. That means the first two weeks of a 40-week pregnancy technically precede conception. If your cycle is consistently longer or shorter than 28 days, the estimate shifts by the difference.
How accurate is the estimate
Only about 4% of babies arrive on the estimated date. Roughly 80% of spontaneous births happen between 37 and 42 weeks. A first-trimester ultrasound dates a pregnancy more accurately than the last-period method, and clinicians will usually adjust the date if the scan differs by more than about a week.
Trimester milestones
- First trimester (weeks 1-13): organ formation; first scan usually 8-14 weeks.
- Second trimester (weeks 14-27): anatomy scan around 18-22 weeks; movement becomes noticeable.
- Third trimester (weeks 28-40+): rapid growth, more frequent appointments, term from 37 weeks.
Key takeaways — Pregnancy Due Date Calculator
- Due date = first day of last period + 280 days, adjusted for cycle length.
- Most births occur within a five-week window around that date.
- An early ultrasound is the most reliable dating method — always follow your clinician's date.