Date Calculator

A due date is conventionally estimated as the moment of conception plus about 266 days (a little over 38 weeks) - or, from the first day of the last menstrual period, plus 280 days (40 weeks), since ovulation/conception typically happens roughly 2 weeks after the period starts. The true population average length of pregnancy varies a little between population groups, which is why this calculator offers an optional race/ethnicity adjustment below.

Only about 4-5% of babies are born on their exact (average) due day. Most babies - around 80-90% - are born within 2 weeks before or after the due date. This is why many people find it more useful to think of the due date as a "due month", or at least a 4-week window, rather than a single fixed day.

Is IVF different? Yes. IVF pregnancies can be dated far more precisely than natural pregnancies, because the egg retrieval date (and the exact age of the transferred embryo) is known exactly, instead of being estimated from a fertile window. Due date = egg retrieval date + 266 days, which is equivalent to day-3 transfer date + 263 days, or day-5 blastocyst transfer date + 261 days.