Pregnancy Due Date

The due date is estimated using Naegele's rule: adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle. If your cycle is longer or shorter, the calculator adjusts accordingly. For the most accurate estimate, an ultrasound performed between 8 and 12 weeks can narrow the window to within 5–7 days.

Only about 5% of babies are born on their exact due date. It is an estimate — a healthy full-term pregnancy ranges from 37 to 42 weeks. Think of your due date as a "due window" rather than a deadline. Your healthcare provider will monitor your progress with ultrasounds and other assessments.

Pregnancy is tracked by counting weeks from the first day of your last menstrual period (gestational age). Most full-term pregnancies last 40 weeks. The first trimester covers weeks 1–12, the second trimester weeks 13–26, and the third trimester weeks 27–40. Key milestones include the first heartbeat at around 6 weeks and the anatomy scan at 18–20 weeks.

If your cycle length is not 28 days, the calculator adjusts the due date based on the number of days in your average cycle. For example, if your cycle is typically 35 days, the calculator adds 7 extra days to your estimated due date because ovulation likely occurred later. However, if your cycles vary significantly from month to month, your due date estimate will be less precise. An early ultrasound is the most reliable way to establish an accurate due date in this case, as it measures the baby's size directly rather than relying on cycle assumptions.

Yes. With IVF, your due date is calculated from the embryo transfer date rather than your last period. A day-3 embryo transfer is treated as equivalent to a gestational age of 3 weeks and 3 days, while a day-5 transfer is treated as 3 weeks and 5 days. Your fertility clinic will provide an exact due date based on your treatment protocol, which is more accurate than any calendar-based estimate. Share your IVF dates with your obstetrician to ensure proper prenatal scheduling and testing timing.