Pregnancy Calculator

How Due Date Is Calculated

A standard pregnancy lasts approximately 280 days (40 weeks) from the first day of the last menstrual period (LMP). This is known as Naegele's Rule. Because ovulation typically occurs around day 14 of a 28-day cycle, the actual time since conception is about 266 days (38 weeks).

Naegele's Rule

Add 280 days (40 weeks) to the first day of the last menstrual period. For cycles longer or shorter than 28 days, the due date is adjusted by the difference. For a 30-day cycle, 2 days are added; for a 26-day cycle, 2 days are subtracted.

The Three Trimesters

First trimester (weeks 1–13): Organ formation, highest miscarriage risk, morning sickness common. Nuchal translucency scan typically at weeks 11–14.

Second trimester (weeks 14–27): Often the most comfortable period. Anomaly scan at weeks 18–21. Baby begins moving (quickening) around weeks 16–20.

Third trimester (weeks 28–40): Rapid growth phase. Baby gains most of its birth weight. Full term is considered 37–42 weeks.

IVF Due Date

For a 5-day blastocyst transfer, the equivalent LMP date is calculated as transfer date minus 17 days (since a 5-day blastocyst is equivalent to 5 days past ovulation, and ovulation occurs 14 days after LMP). For a 3-day embryo transfer, subtract 19 days from the transfer date.

How Accurate Is the Due Date?

Only about 5% of babies are born on their exact due date. Most are born within 2 weeks either side. Ultrasound measurements in the first trimester are the most accurate way to confirm gestational age, often adjusting the LMP-based estimate.