|
Overview :
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).
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.
|