Why do they count your pregnancy from the first day of your last period?

Question

Because I know they day I conceved. And it was almost 2 weeks after that. Just curious.

Answer

You might know the day you conceived, but it's impossible to know when the fertilized egg implanted, it can be as fast as two days or as long as 7+.. Ya just never know!

Answer

You are technically only pregnant for 38 weeks but it's counted as 40 because they add the 2 weeks from lmp

Answer

Doctors do it because it's a reliable way to date all pregnancies. Most women don't know the day they conceived, but basically all women clearly know the first day of their last menstrual period. 
If they counted from the last menstrual period for some women and date of conception for others, it would get confusing fast. So they standardize everyone to the date of the last menstruated period.

If you want to refer to the number of weeks since conception, they call that the fetal age instead of the pregnancy age.

Answer

I know the day I ovulated and had implantation bleeding a week later. My doctor says I'm 7 weeks but I'm following along with development at 6 weeks since I know he dates everything happened. My ultrasound is in 2 weeks and will confirm the exact info though, so I'm waiting until that to change my  baby development tracker.

Answer

It's about 2 weeks for everyone, usually you ovulate 12 days after cycle day 1. They count it from there because it was the start of a new cycle and the cycle in which you conceived.

Why do they count your pregnancy from the first day of your last period?

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