The short answer is YES. It’s not easy to get pregnant if you have sex during your period, but it is possible since pregnancy does not begin until a few days after sex. Menstruation is the end of your menstrual cycle and represents the loss of an ovum (an egg) that could have been fertilized during several days to two weeks earlier, but was not fertilized. Whereas the ovulation-menstruation cycle is roughly 28 days on average, there is actually enormous variation among women. Indeed, it is on account of such wide variation that we know that human ovulation is not related to cycles of the Moon. Your cycle can be much longer, or much shorter than a month, and, if it is much shorter, then it is possible for you to ovulate fairly soon after your period ends.
Perspective on human pregnancy must take into account the fact that, over all, it is difficult for human women to get pregnant. Pregnancy begins when an embryo –a fertilized ovum– implants in the uterus, which occurs during what is called the third gestational week. That’s when it takes place, on average, since the weeks of pregnancy are counted based on the time since the last menstrual period. When it comes to getting pregnant, timing is everything. Usually, there is no ovum available to sperm, so you won’t get pregnant following sexual intercourse. However, there is a fertile window, a period that is usually about 4 days long surrounding ovulation, during which sperm can have the opportunity to meet an egg that is ripe for fertilization. Now, even if a woman has ovulated, there is only one egg, and to reach that egg, a sperm cell must swim. For fertilization to occur, a rendezvous between ovum and sperm must happen in the fallopian tube. To swim, each sperm is equipped with a tail, but the road is full of obstacles, from acidity of the vagina to all the twists and turns in the pathway to the fallopian tube. Most sperm cells don’t make it. In the end, only about one hundred sperm come into contact with the ovum. Furthermore, there is another obstacle, the zona pellucida, a protective barrier around the ovum. To break though, the sperm cells release enzymes. This enables one sperm, and only one, to get its nucleus inside the ovum. That nucleus contains the father’s genome –genetic instructions for making and controlling his cells. However, the ovum has a nucleus of its own, and it contains the mother’s genome. The two genomes merge, creating a unique cell, with a distinct genome.
Once an ovum has been fertilized, it is called a zygote, a cell that soon divides into two cells, four cells, and an increasing number of cells. As it divides, the mass of dividing cells continues its journey through the fallopian tube. Five to six days after fertilization, the dividing cells have arranged themselves into a blastocyst, which includes an inner cell mass and a fluid-filled cavity, surrounded by an outer layer of cells. If the blastocyst implants in the endometrium, the lining of the uterine wall, at that point you are actually pregnant.
During the time between sexual intercourse and fertilization of the ovum, although the number of sperm that remain viable decreases rapidly, especially after the first day or two subsequent to sexual intercourse, there is a bell curve of sperm viability, and many sperm can live up to three days, and sometimes even as long as five days. Thus, if you have sex during your period, and particularly toward the end of your period, it is possible for enough sperm to remain viable enough to achieve fertilization in the event that you happen to ovulate extremely early.
Because of this possibility, if you do not wish to become pregnant, it is advisable that you use contraception, even when you are having your period. Now, if you are on an oral contraceptive regimen in which there is a 7 day period at the end of the cycle with no pills (or with hormone-free pills), that is alright, since you are on a contraceptive treatment. However, if you use a barrier method, such as a diaphragm or condom, you are not protected if you do not use the method during your period. As for the practice of coitus interruptus as a method of birth control, see our The Pulse blog about this topic regarding its history and effectiveness. The take home message is that it works a little bit, but is not very reliable. Thus, even while having sex during your period, you should use a different method, such as oral contraceptives, a barrier method, implanted hormone devices, or an intrauterine device (IUD). Unless, of course, you wish to become pregnant.