How high are the chances of getting pregnant if a guy ejaculates then puts his penis back in the vagina ?

If unprotected: semen will continue to flow from his penis, especially if intercourse is resumed: yep- you can get pregnant: double dipping= not a great idea! Do a home pregnancy test.

Pregnancy: Fertility rates average about 20% per cycle. If no form of protection is used than this puts you at significant risk to become pregnant. Always use protection if you don't wish to become pregnant.