Due Date Calculator

The most common method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's rule: add 7 days to your LMP, then add 9 months (or subtract 3 months). For example, if your LMP was January 1st, add 7 days = January 8th, add 9 months = October 8th due date. This assumes a 28-day cycle and ovulation on day 14. Only about 5% of babies arrive on their exact due date—most healthy deliveries occur between weeks 38-42.

Several factors can cause differences: Irregular cycles affect LMP-based calculations (cycles longer than 28 days mean later ovulation). Ultrasound dating in the first trimester (weeks 8-12) is often more accurate, measuring the embryo's crown-rump length. If ultrasound differs by more than 5-7 days from LMP calculation, doctors typically adjust the due date. Conception date is rarely known exactly. Your healthcare provider uses clinical judgment combining all available data.

Pregnancy divides into three trimesters: First trimester (weeks 1-13): Major organs form, highest miscarriage risk, morning sickness common. Second trimester (weeks 14-27): Often called the 'honeymoon period'—more energy, baby movements felt, anatomy scan at 18-22 weeks. Third trimester (weeks 28-40): Rapid baby growth, prenatal visits increase, birth preparation. Understanding trimesters helps you know what prenatal tests and milestones to expect and when to schedule important appointments.

IVF due dates are typically more accurate because the exact fertilization date is known. For Day 3 embryo transfers, add 263 days (37 weeks, 4 days) to the transfer date. For Day 5 blastocyst transfers, add 261 days (37 weeks, 2 days). This eliminates the uncertainty of ovulation timing. However, the due date is still an estimate—delivery timing varies based on individual factors. Your fertility specialist will confirm dating with early ultrasound measurements.

Medical definitions have refined 'term' pregnancy: Early term (37-38 weeks, 6 days), Full term (39-40 weeks, 6 days), Late term (41-41 weeks, 6 days), Post-term (42+ weeks). Research shows babies born at 39-40 weeks have better outcomes—fewer breathing problems, better feeding, and stronger brain development. Labor induction before 39 weeks without medical necessity increases risks. After 42 weeks, placental function may decline, so most providers recommend delivery by then.