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

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.