Generally, chances of getting pregnant during the menstruation or periods is little less but it depends on the release of the egg from the ovaries. During ovulation, the ovaries releases an egg. A woman ovulates 14 days before her next periods and this is the time when she can easily conceive.
After ovulation, the egg moves toward the uterus through the fallopian tubes and if there is any sperm, the egg fertilizes with the sperm. It is not necessary that the sperm will not survive in the uterus. The average life span of a sperm is for 2-3 days!
When you have your period, another egg is developing in preparation for release during the current cycle. Some women can have 28 days long cycle or just 22 days. The shorter the cycle days, the earlier the ovulation starts. If the sperm is in the tract and you start ovulating within 3 days, you can get pregnant easily during menstruation.
Sometimes, there can be no ovulation and therefore, the ovaries don't release the egg. Even after missing ovulation, a woman can have her regular menstrual cycle. But an unprotected intercourse during or immediately after the ovulation can make you get pregnant.
So, depending on the ovulation and egg maturity, your chances of getting pregnant even during your periods or menstruation varies. Ito be on the safer side, avoid unprotected intercourse and if you are planning to conceive, the best time is during the ovulation. You can either check the ovulation calender or use an ovulation strip to keep a track of the most fertile dates.