Why do we calculate the length of a pregnancy from the first day of a women’s last period?
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Before ultrasounds existed, it was the best way to get a theoretical date of conception. From their calculations in the early 1900s, the average woman ovulated on Day 14 of her cycle. The cycle starts the first date of her last menstrual period (LMP). The child cannot be conceived prior to the woman ovulating, so identifying when she ovulated is key to understanding when she conceived. Since the date of ovulation/conception is calculated from the LMP, they just run with that.
It’s a little silly because when you are four weeks pregnant, the zygote/embryo has only existed for two weeks. It’s basically a historical artifact from a time when we knew very little about women’s fertility.
We have now studied female anatomy enough to know that this is only a very rough estimate. Women don’t necessarily ovulate on day 14. Women may have long or irregular cycles. If you live in a developed country, they will often revise the expected due date based on an ultrasound or on urine/blood testing if they look more than one week off. I use Natural Family Planning to track when I actually ovulate and my doctors/midwives have been willing to revise my due date based on that – so long as the other data fits.
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