Some people think that if they have unprotected sex during their period they cannot get pregnant. The assumption is that since ovulation is clearly not happening, all is safe. This is NOT true.
THE REAL STORY:
Step 1: Millions of sperm swimming in semen exit a man through his penis and enter a woman’s vagina, travel through the cervix, into the uterus and up the fallopian tubes.
Step 2: The sperm cells stay alive and active inside the woman – sometimes for days. Research shows that sperm can remain active for five to seven days.
Step 3: The woman's period ends and her body releases an egg (she ovulates). Most women, but not all, ovulate seven days after their period is completed. Some women ovulate early.
Step 4: The sperm, still active, meet the egg that was released early and an embryo is created.
OTHER CONSIDERATIONS BESIDES PREGNANCY
Physical risks: Period sex is obviously bloody. STDs are spread through blood. IPeriod blood (or any blood really) greatly increases the chances of her partner getting an STD if she has one. A woman is also more susceptible to an STD during her period for several reasons:
During a woman's period her cervix is more open which can allow STDs to enter more easily
The acidic environment of the vagina protects against infections. The vagina becomes less acidic and more alkaline during a woman's period, creating a more welcoming environment for STDs
Skin is the number one barrier against infection. Breaks in the skin increase the risk for infection. During her period woman's vagina has less natural lubrication. Less lubrication means the vagina is more at risk for tears and breaks in the skin that STDs can get into.
Spiritual Considerations: Many unmarried Jewish people do not realize there is a beautiful tradition that Jewish couples have been keeping for thousands of years since the time of Avraham and Sarah to not have sex when a woman has her period.
There is a major halachic prohibition against having sex during your period, or with someone who has their period. This is so serious that rabbis instituted a seven day waiting time after a woman's period has ended, before any acts of intimacy can resume. Most importantly, married women do not resume intimate contact until they have gone to the mikvah. The Clouds of Glory that are said to have always rested over Sarah’s tent was a reference to mikvah (cloud = rain water). If you are having sex outside of marriage but want to try and follow Torah law, please consider:
1. Making sure your period has fully stopped before having sex.
2. Even though mikvah is a ritual for married women, some halachic authorities hold that a sexually active unmarried woman should go to the mikvah as well. Going to the mikvah can be at a official mikvah or any large body of natural water such as the ocean.
A rabbi whom you feel comfortable talking with about this matter can provide further guidance.