I don't ever like to suggest sex without some sort of protection so it really isn't safe.
Biologically, during the period the womb lining is being flushed out and so are any eggs produced with it, however eggs can stay in the tubes and sperm can get up there so you are never 100% safe to have unprotected sex.
If you and your girlfriend don't want to get pregnant, don't have unprotected sex.
sperm can live in the womans body for days after you have sex, waiting for her to ovulate. also younger women have irregular ovulation, you just don't take the risk!
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