Obstetricians usually date pregnancies in weeks, typically from the first day of the last menstrual period. Since fertilization occurs 2 weeks after this date, after a new ovum is released, obstetric dates are about 2 weeks longer than embryologic dates.

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))

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