If a woman is hoping to get pregnant, it's commonly assumed that her best chance falls sometime around 14 days after her last period [source: Mayo Clinic]. But that rule of thumb only applies for those with a textbook 28-day menstrual cycle. More generally, women may reach peak fertility between 11 and 21 days after their periods, depending in part on the frequency and duration of their menstruation. Irregular periods that only come around every few months, as well as cycles shorter than the average 28 days may hold higher probabilities for overlap between ovulation and menstruation. Although the statistical chances of it happening are low, there are three main ways that it's possible to get pregnant from unprotected sex despite the presence of vaginal bleeding [source: American Pregnancy Association].
First up, if a woman has an especially short menstrual cycle, ovulation can occur before menstruation stops [source: Hirsch]. This is the least likely scenario for a period pregnancy to take place, but a longer period lasting up to seven or eight days combined with a compressed menstrual cycle can add up to an embryo. More commonly with shorter menstrual cycles, the ovaries could release an egg a few days after menstruation finishes. But even though an egg wasn't ready and waiting during the period itself, unprotected intercourse on, say, the last day of bleeding could leave behind sperm with a surprisingly potent shelf life. Sperm can survive inside a woman's body for three to five days, possibly resulting in pregnancy if the ovaries release an egg into the fallopian tubes in that time frame [source: American Pregnancy Association].
Vaginal bleeding can also be misleading, as it doesn't always signal menstruation. In fact, some women spot during ovulation, which could be mistaken for a light period [source: Barton]. In that case, unprotected sex could certainly result in pregnancy. And whether pregnancy prevention is a concern or not, having sex without a condom or other type of barrier method still puts partners at risk for contracting sexually transmitted diseases. Safe sex, in other words, is the smartest sex no matter what time of the month it is.