Why do pregnancies start counting from the last day of your period instead of the day of conception?


Also, how accurate is the due date?

In: 801

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.