Table I-4-2. Pregnancy DatingDuration of pregnancy using: Menstrual dating 280 days or 40 weeks
Assumed cycle length 28 days
Calculate due date Naegele’s rule Last menstrual period – 3 months + 7 days
Basal body temperature: The rise in basal body temperature (BBT) is
assumed to be caused by the thermogenic effect of progesterone produced by
the corpus luteum that formed after ovulation. The accuracy of BBT is ±1
week.
Menstrual history: Menstrual dating assumes ovulation occurred on day 14
after the first day of the LMP. However, normal menstrual cycles can vary
from 21 to 35 days, making ovulation possible on day 7 to day 21. Because
most women’s cycles are more or less than 28 days, adjustment of the due
date may be necessary. Accuracy of menstrual dating is variable depending on
the patient’s memory and record keeping. The accuracy of menstrual history
is ±1 week.http://ebookobgyne.net