When did you have to stop having sex in the missionary position?

CC
I.e. Him on top.  
How many weeks along?  
Update: thanks for the responses.  
To clarify, I understand "missionary" is HIM on top.  
When you are on top facing him it's called "cowgirl" (riding him 😉) 
When you are on top facing his feet (your back to him) it's called "reverse cowgirl".  
The position I'm asking about specifically is missionary.