Why do we calculate the length of a pregnancy from the first day of a women’s last period?
In: 3216
It’s the most consistent measure for the general population.
Ovulation usually occurs about 14 days after the first day of the period. Some women have irregular cycles or anovulatory cycles where they don’t release an egg at all, so relying on an ovulation date is not realistic except for in rare cases where hormones or basal body temperature is closely monitored.
Additionally, sperm can live in the vagina/uterus for 5-7 days, so even if there was a one-off event where the day of intercourse is definitely known for that pregnancy, that day is not necessarily the date of conception.
The best way to get a consistent expected delivery date is the first day of the last menstrual period, so that’s the dating system we use. Even if the due date is established by first trimester ultrasound, the weeks of gestational age are still assigned by convention with day 1 being the first day of LMP.
Latest Answers