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

Question

Also, how accurate is the due date?

In: 801

Answer 1

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.