Here are a few reasons why:

1. Sperm Survival: Sperm can live inside the female body for up to five days. If a woman has a short menstrual cycle or if her period lasts longer, she could ovulate soon after her period ends. If sperm is still present from intercourse during her period, it could fertilize an egg if she ovulates early.
2. Irregular Ovulation: Not all women ovulate in the middle of their cycle. Some women might ovulate earlier or later, and some might even have unpredictable ovulation patterns. This unpredictability can increase the chances of pregnancy if intercourse occurs during a period.
3. Spotting: Sometimes what is believed to be a period is actually spotting, which can sometimes occur during ovulation. If a woman mistakes this for her period and has unprotected sex, she could get pregnant.
4. Short Cycles: Women with shorter menstrual cycles are more likely to have a fertile window that is closer to their period.

It's important to note that while the chances might be lower during menstruation, there is no time in the menstrual cycle when the risk of pregnancy is zero if you're having unprotected sex.

Additionally, it's worth mentioning that having unprotected sex at any time increases the risk of sexually transmitted infections (STIs). If you're sexually active, it's crucial to use protection and have regular check-ups with a healthcare professional.

If you're trying to avoid pregnancy, it's essential to use a reliable form of contraception consistently and correctly. If you have questions about birth control methods, it's a good idea to consult with a healthcare provider.