How long does a pregnancy last? ~ Almost a dad

How long does a pregnancy last?

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

The length of a typical pregnancy is approximately:
- 38 weeks from time of conception
- 40 weeks from last menstrual period (or LMP as some call 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.
 To sum up, doctors talk about pregnancy on a 40 week timeline, the actual gestational time is about 38 weeks. All the being said, there are some variances in baby due date accuracy.