If You Are Already 4 Weeks Pregnant, Can Someone Else's Sperm Reach The Fetus?

Once you're pregnant the egg is already fertilized. It can't be double-fertilized by two different men. I'm not exactly sure what you're asking except that it sounds like you had sex with a man who is not the father of your baby and you want to make sure that the father of the baby isn't going to change just because he shot some seed up there. No, it won't. But when you're this early in a pregnancy you should be extra careful about having sex and if you're not monogamous make sure he bags it. The last thing you want when you're pregnant is an STD.

It would not affect the fetus after the initial fertilization the cells skins harden so that no other sperm could penetrate of course there is the accidental twins where two sperm get in at the same time but by four weeks your safe no other sperm can affect your fetus

No. The cervix usually closes the door until the celebration of birth happens. It is the bodies way of keeping the fetus safe from infection and disease.

Actually no.  The only way to get twins is to have the egg fertilized by two sperms at the time of conception or by having the zygote divide into two different zygotes at the time of conception. ( That's the case when you get identical twins. ) Once the moment of conception occurs, your body protects the egg from every other sperm and anyway, fertilization occurs only once for each egg if any. Take care!

Non identical twins are from a different cell sperm, and they do not share the same placenta. They do not have the same chromosomes...