Also, how accurate is the due date?
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IIRC they actually count from the first day of your last menstrual period, not the last. Reasoning for that is that people bleed for different lengths of the time.
And the main reason that we don’t count from date of conception is that most people don’t know when that was. While we all go through the same phases of our cycle, how long each phase lasts is individual, and not everyone logs when they have sex. That makes is very difficult to calculate exact date of conception, so they use the most definite last date they have to go by.
You’ll be given a due date based on LMP when you first present to the doctor’s office, but that date is adjusted (in Canada at least) sometimes when the anatomy scan is done around 20 weeks based on the measurements they do which can more accurate predict how old the fetus is, but it usually doesn’t get altered by much. However a lot of factors can determine if baby will be born on their due date. I read a statistic some years ago that only about 5% of babies are actually born on their due date.
It all comes down to reliable information.
If you find an OBGYN who regularly works with moms who practice Natural Family Planning (NFP) and keep good logs. (or something similar) Those Dr’s offices often use conception date instead of, or in conjunction with, the date of the last period.
A good amount of couples don’t know exactly when they are fertile, so the last period is a more reliable data point for many people, and it’s close enough to accurate to provide good healthcare based on that data alone.
TLDNR: Conception day is more accurate but harder to know and the last period date is good enough.
Because it was at the time (and probably still is) nearly impossible to accurately tell the day of conception. Even if you only had one partner you had sex with one time, the actual date that the ~~spent~~ *sperm* fuses with the egg can lag by as much as a week.
When you’re trying to conceive, you’re likely having sex multiple times a month, so it’s very hard to nail down the probable conception date. It’s much easier to use the first date of your last period to estimate the due date.
It probably *should* be counted from the date of fertilization, but that’s pretty much impossible to determine, so the last period date is a decent guess.
And the due date is nothing more than an educated guess as well, so it doesn’t really matter very much for any practical reason.
Not every woman knows which day she got pregnant. What’s sad is some get halfway through the pregnancy before they realize they are pregnant.
I recall reading in “A Thinking Woman’s Guide to Pregnancy” that it’s because a French doctor in the 1800’s observed that it’s roughly 40 weeks from last period to birth and his own confirmation bias turned it into a rule. He was an authority on obstetrical care, and “taught” his rule to others who adopted it. One of the many situations in which a person’s clever idea becomes conventional wisdom and medical “truth” without actual scientific scrutiny. It’s been almost 12 years since I was last pregnant, and I don’t know where I put the book. Maybe someone can correct me, but I believe the statistics show average first pregnancies are 41 weeks and subsequent pregnancies are 40.5.
Of course today there’s the political complication of making it sound like a person seeking an abortion has been pregnant and “should have known” earlier than she did. It’s quite obvious to me that we can’t be pregnant before we had sex, so estimating ovulation from the last period, then counting pregnancy weeks from ovulation or the first day of the missed period seems more realistic to me.
The easiest answer is because it’s very, very rare to pinpoint the date of ovulation (the point at which an egg is released), and therefore conception. And that’s only if someone is trying with multiple methods. The majority of people do not know when they ovulate. Also, after ovulation, the egg can live for up to 24 hours. The variations cause too much confusion, so to maintain consistency, they go by last menstrual period.
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