Fun fact: Catholics are more likely than many other groups to have period sex, possibly because of church prohibitions on using contraceptives.
When it comes to blood-borne diseases such as HIV, it would not be surprising to find that having period sex could increase the risk of HIV transmission. The virus is, after all, found in the blood, and during menstruation there is a lot more blood to which a woman's sexual partner may be exposed -- even if they are practicing safer sex.
In fact, there is a small, if controversial, body of research suggesting just that -- that sex during menstruation, as well as other exposures to blood during sex, may increase the risk of heterosexual HIV transmission. There is, however, also some evidence that sex during menstruation is associated with an increased risk of other sexually transmitted diseases.
Why would STD risk increase during period sex for STDs that are not transmitted through blood? Well, there are several theoretical reasons why you might see such an increase:
- The amount of the pathogen varies with the menstrual cycle.
This explanation is plausible, but not proven. In at least one study, the amount of shedding of cytomegalovirus (CMV), but not herpes, has been shown to vary cyclically. However, the maximum shedding time was during the luteal phase, not during menstruation.
- The blood flow acts as a carrier for viruses and other pathogens.
Physiologic changes during menstruation could also increase a woman's susceptibility to infection, and menstrual blood could enhance bacterial growth.
- Because a woman's cervix is more open, she may be more susceptible to upper cervical and uterine infections.
There is conflicting data on whether ascending infections and pelvic inflammatory disease (PID) are associated with sex during or near the time of menstruation -- it may be more folk wisdom than scientific fact. It is, however, worth mentioning that an alternative explanation for why ascending infections might be more common in the week after menstruation could be that existing infections are be more likely to ascend into the uterus and become symptomatic PID during menstruation, even if the sexual act that caused them occurred at another time.
Women who have period sex are likely to have more sex and more sexual partners -- increasing their risk in a way that has nothing to do with menstruation.
Although there is some research that suggests that women who have sex during their period are at increased risk of STDs other than HIV, there is also data showing that women who have sex during their periods are simply more sexually active, and have more sexual partners, than women who choose to abstain from sex during menstruation. This could mean that it's not having period sex that increases the women's STD risk, but the fact that they're higher risk in general.
- Exposure to menstrual blood causes skin irritation and inflammation
that might increase susceptibility to infection.
Menstrual blood can be an irritant for some people, and skin irritation could increase susceptibility to various STD infections. There is also some data that women may be more susceptible to vulvar skin irritation during their periods. Finally, menstrual blood can also dilute the effects of both natural and artificial lubrication, potentially increasing the risk of tearing and other skin damage.
In the end, the data on period sex and STD risk isn't all that clear. While self-reported data does suggest that period sex is associated with an increased risk of STDs, more rigorous studies have not found as clear-cut results. Although I'm not convinced that anyone is ever going to put the time and effort into doing the research, it might well turn out that the association between STDs and period sex is more about the type of people who have sex during menstruation than the absolute risk of having sex while a woman is bleeding.
That said, if you're going to have sex during menstruation, it's certainly not safer than doing so during other times of your menstrual cycle, and so it's still a good idea to use condoms and take other safer sex precautions... not to mention precautions to keep from getting blood stains on your mattress. Those things are crazy expensive to clean.
Agner T, Damm P, Skouby SO. Menstrual cycle and skin reactivity. J Am Acad Dermatol. 1991 Apr;24(4):566-70.
Farage M, Warren R, Wang-Weigand S. The vulva is relatively insensitive to menses-induced irritation. Cutan Ocul Toxicol. 2005;24(4):243-6.
Foxman B, Aral SO, Holmes KK. Interrelationships among douching practices,risky sexual practices, and history of self-reported sexually transmitted diseases in an urban population. Sex Transm Dis. 1998 Feb;25(2):90-9.
Filer RB, Wu CH. Coitus during menses. Its effect on endometriosis and pelvic inflammatory disease. J Reprod Med. 1989 Nov;34(11):887-90.
Lurie S. Does intercourse during menses increase the risk for sexually transmitted disease? Arch Gynecol Obstet. 2010 Dec;282(6):627-30. Epub 2010 Jun 25.
Mattson CL, Bailey RC, Agot K, Ndinya-Achola JO, Moses S. A nested case-control study of sexual practices and risk factors for prevalent HIV-1 infection among young men in Kisumu, Kenya. Sex Transm Dis. 2007 Oct;34(10):731-6.
Mostad SB, Kreiss JK, Ryncarz A, Chohan B, Mandaliya K, Ndinya-Achola J, Bwayo JJ, Corey L. Cervical shedding of herpes simplex virus and cytomegalovirus throughout the menstrual cycle in women infected with human immunodeficiency virus type 1. Am J Obstet Gynecol. 2000 Oct;183(4):948-55.
Tanfer K, Aral SO. Sexual intercourse during menstruation and self-reported sexually transmitted disease history among women. Sex Transm Dis. 1996 Sep-Oct;23(5):395-401.