The length of a pregnancy may sound like a basic question, but one worth asking as there is often some confusion around it.

Why are there two different lengths? 

Doctors use the LMP date as it is often the most solid point of reference. Many people don't know the actual date of conception, so the LMP allows for consistent comparisons from person to person amond medical professionals. The 38 week timeline is often referred to as the gestational age.

I thought a pregnancy was 9 months long? Isn't 9 months 36 weeks?

Well, it is 9 months, but the average month is 4.3 weeks long, not 4.

- (52 weeks / 12 months = 4.333 weeks per month.)  

- 4.333 weeks x 9 months = 38.99 weeks

So 9 months is just about the average of the 38 and 40 week numbers.