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

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.