During menstruation a woman is more likely to get an infection during unprotected sex.
If you are thinking pregnancy, then it depends on the woman's cycle. If she has a 30 day cycle, then you are safe but if she has a 23-24 days cycle then her ovulation is about to occur on about 9th day of the cycle, meaning 9 days after her period started.
So, if you have sex on the 5-6th day of her period and you come inside of her and sperm can live up to 5 days inside the woman, it means she could easily get pregnant because spermatozoa will be alive when her ovulation occurs.
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