Dating Pregnancy from the Last Menstrual Period

Pregnancy is considered to last 266 days from the time of conception, or 280 days (266 + 14) from the first day of the last menstrual period (if menses are regular at 28 days).

estimated date of confinement (EDC) =

= DATE((first day of last menstrual period) + 280)

Nagele's rule states that the estimated date of confinement (EDC) can be approximated  as:

estimated day of confinement = 

= (first day of the last menstrual period) - (3 months) + 7 days

The EDC is only an approximate date, with only 10% of patients actually delivering on the EDC. It is more accurate to report the EDC +/- 2 weeks, as delivering 2 weeks before or after the EDC is within the normal range.

To calculate the week of gestation at a given date:

week of gestation =

= INT((date) –  (date for LMP+14)) / 7)

extra days =

= ((date) – (date for LMP + 14)) – (7 * (week of gestation))