For a while, I always thought that you could have unprotected sex while you had your period, and it was a "safe" time when you wouldn't have to worry about getting pregnant. Then I heard there are a few cases where women have gotten pregnant, so what's the deal?!
If a woman has a typical 28-day menstrual cycle, then she will ovulate in the middle of her cycle (around 14 days after the 1st day of her period). Even if she has unprotected sex 5 days before she ovulates, she can get pregnant (remember that sperm can live up to 5 days inside a woman's vagina).
Studies show that if a woman's cycle is shorter than 28 days, she'll ovulate much sooner, so if she has unprotected sex while she has her period, it's possible that sperm can still be around when she eventually ovulates - meaning women with shorter cycles may be able to get pregnant if they have unprotected sex while they have their period.
Other factors also play a big role. Stress and certain medications can make you ovulate later or earlier than expected, which in turn will affect when you get your period. This can cause some women to have erratic cycles, where one month it's 25 days long and the next month it's 33 days long. If this sounds like you, then it may be hard to determine when you ovulate, so if you don't want to end up preggers, don't have unprotected sex ever.
What about a "false period?" To find out
A true period happens about 2 weeks after you ovulate. The blood is a shedding of your uterine wall due to lack of conception. A false period, like spotting can really be the first signs of pregnancy so if she doesn't know she's pregnant, she can continue going through the month not knowing she's already pregnant -- a scary thought I know. If she had sex during that "false period" when she was spotting (and already pregnant), she'll think she got pregnant at that time, when in reality, she got pregnant about 2 weeks before that "false period."
Spotting or irregular bleeding (that's not a real period) can also happen if you're using an IUD, if you've recently had an abortion, if you have stress or a fluctuation in your hormones, if you have low thyroid levels, injury to your vagina, an infection, or if you are on certain medications such as anticoagulants.
The bottom line is, unless you are charting your cycle every day and measuring your fertility signs (cervical fluid, body temperature, and position of your cervix), there's really no way to know for sure when you ovulate and if your period is a true period.
Dear's Advice: If you are sexually active, and you don't want to get pregnant, make sure to use an effective form of birth control every time you have sex. It doesn't matter if you use condoms, the Pill, the Nuvaring, the Patch, or the Shot - just make sure to use it properly!