Also, how accurate is the due date?
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I’d wager it’s because it was the most accessible way. Women will always have been aware of their cycle, and without modern tools such as ultrasound or blood testing its a useful data point. Yes there will be variables but an average cycle is 28 days.
Then once the pregnancy is older you can use things like fetal heart rate and size on palpation to confirm.
The due date takes this into account
The egg starts developing and the placenta starts forming independent on if or when the conception takes place. The conception can happen at any time and does not have any impact on when the delivery is to take place. We start counting from the last day of the period because this is the most accurate way of measuring.
The due date is in a healthy pregnancy accurate to within a week. Most will find it accurate to a day or two. But of course not all pregnancies are fully healthy and you can have premature delivery or late delivery.
It’s the date that’s able to be most reliably known.
If someone is having sex regularly there are multiple possible dates of intercourse, so that’s often not useful. Conception can happen any time between one and five days after intercourse, so no way to know that date either. Implantation can also happen over about a 5 day period, depending on when fertilization occurred. The first “missed” period is another possible marker, but relies on a woman having a reliable cycle and even then as it never happens is a guess anyway.
So to have a marker they use the one known date as a “good enough” counting point.
Accuracy is hard to nail down to the day, but generally not bad for picking the week.
If a couple of trying for a baby and having sex every day, they can’t know which day was successful, so you go off a day you know for sure. That gives doctors a rough estimate of how far along the mother is, until they can do other tests to get a more accurate picture, such as ultrasounds.
Between the known date of their last period and visual evidence of the fetuses development, they can then make a pretty accurate guess on how far along the pregnancy is and when they expect the due date to be.
When you get pregnant, there are several important dates:
LMP: Last menstrual period
Insemination Date: When the sperm got into the egg
Implantation Date: When the egg+sperm combo (called an embryo) implanted into the uterus
Problem is, unless you’re doing fertility treatments, Insemination and Implantation dates are hard to tell – LMP’s good enough! Estimated Dates of Delivery (EDDs) are generally accurate to within a week or two off of the LMP, and can vary depending on personal biology and just, you know, when things are ready.
When doctors do an IVF transfer and manually stick the sperm into the egg, and manually stick the embryo to the uterus, they do, in fact, know those precise dates, and Estimated Date of Deliveries are based off those, with a much higher level of precision (but still some biological variability), and the LMP backfilled if not known off of that information.
Because the date of conception is not known. The date of your last period is known so we go by that.
You could go halfway through the menstrual cycle but that would also be a guess.
People here have mentioned that you can’t know when the baby was conceived if you have sex every day but that’s irrelevant. You could have sex only once and that still wouldn’t tell you when the baby was conceived.
Sperm cells can live inside the vagina for many days, up to 5 or so. They need to hang around until they touch the egg and fertilize it. Then the egg hangs around for a few more days before implanting in the uterus.
The moment of fertilization must be precise, it can’t happen too early or too late because then the egg won’t implant.
And sperm takes time to reach the proper place, so it takes several days from sex to the moment the egg is fertilized.
You can’t know when it happens exactly, it depends on the moment the egg is released and when it’s fertilized or when it implants.
It could happen with sperm that was ejaculated 2 days earlier or 5 days earlier. And the egg doesn’t implant immediately after fertilization. It takes actually another week before that happens. And if it doesn’t implant, there’s no pregnancy.
https://my.clevelandclinic.org/health/articles/11585-conception#:~:text=Within%2024%20hours%20of%20ovulation,cycle%2C%20you’re%20pregnant.
Piggyback on this what happnes if you have irregular periods? Counting from a date three months before actual conception doesn’t seem like what they do.
I saw a really good analogy for it on Reddit that helped me understand. Imagine a woman’s body like building a house. Once a month it builds a house. If no one comes to stay in that house, they tear it down (menstruation)
If someone comes to stay in that house, the house stays up with the person in there. When someone asks you how long that house has been up, would you say since someone has been there? No you would start from when it was built ..
Something like that, I may have botched it lol
Regarding the accuracy part, you need to clarify what you mean by that. Is it the median date (50% probability they baby is born on that day or later, 50% earlier)? Yes, quite accurately.
But the probability that the baby is born on that day is only 4%. And it goes down on the other days (before and after) like a Gaussian (truncated because at some point we take them out).
There is uncertainty in ovulation, intercourse, fertilization, and implantation steps. The one thing there isn’t uncertainty in is the first date of the last menstrual period. If you use that date, human pregnancy is on average 40 weeks long. It would be impossible to calculate a gestational length any other way consistently.
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