Why when my boyfriend {ejaculates} in me it comes back out,
Can this stop me from getting pregnant

Are you trying to get pregnant? Or is this his way to assure you that you won't get pregnant?

No, this won't stop you from getting pregnant.

May I ask how old you and your boyfriend are, and are you financially and emotionally ready to be parents?

No, having his ejaculate leak out will not prevent pregnancy.

Since this was posted in Child and Teen Health, I have to assume you are underage. Having sexual intercourse while underage can result in a whole host of problems including pregnancy, rape charges etc,

No one should engage in sexual intercourse unless that are financially, emotionally and physically ready to have a child.

How old are you ? Are you using any forms of birth control