A reader submitted the following question:

“What is more probable in a random heterosexual encounter without protection: getting an STD or getting pregnant? I assume the answer will change with age (higher chances of STD for older people, higher chances of pregnancy for younger women) but I think would be interesting to know the answer for an average person.”

Thanks for this very interesting, but very complex question! There’s not a simple answer because there are a lot of unaccounted for variables here. I know you mentioned age as one factor that would affect the outcome, but something else you should keep in mind is whether the woman in this encounter is near ovulation or not. The odds of pregnancy will depend upon where she is in her menstrual cycle.

Also, the odds of contracting an STI really depend upon which specific STI you’re talking about, as well as which partner you're talking about. Some STIs are more easily spread from men to women rather than the other way around (e.g., herpes, HIV), so STI risk isn't necessarily equal across the sexes.

Furthermore, it also depends upon whether the person with the infection is symptomatic and, also, whether the infection is being managed/treated. For example, the odds of passing along a genital herpes infection are much higher when the infected partner is having an outbreak. Likewise, the odds of passing along HIV are much lower when the infected partner is taking anti-retroviral medication.

With those caveats in mind, here’s what I can tell you:

When considering a single act of unprotected vaginal intercourse between a man and a woman, researchers estimate that the odds of pregnancy are 21-25% if she is between one and three days away from ovulation. However, the further you get from that ovulatory window, the lower the odds become.

Specifically, outside of that three-day time period, the odds of conception are about 5% when averaged across the rest of the cycle.

For comparison purposes, researchers have estimated that the odds of HIV transmission during a single act of unprotected vaginal sex is 0.08%. This is the risk for male-to-female transmission (female-to-male is lower) for persons in developed countries, and it assumes that the infected partner is asymptomatic and no other STIs are present.

I don’t know how much stock to put in this number, though, because odds of transmitting HIV or any other STI for that matter are very difficult to gauge. There are a lot of assumptions built into these projections, and there a lot of individual circumstances that could increase or decrease that number.

I haven’t seen any studies that estimate the single-act odds of STI transmission for any infection other than HIV. However, I suspect that the numbers are likely much higher for other STIs, such as genital herpes or HPV, which are far more prevalent in most parts of the world and can be spread by simple skin-to-skin contact.

To sum things up, here’s the best answer I can offer to the question originally posed (again, keeping in mind the limitations of the available data and estimates): during a single act of unprotected heterosexual intercourse, the odds of pregnancy are generally going to be higher than the odds of contracting HIV. However, if you look at other STIs (e.g., herpes, HPV), the odds of contracting them may or may not be higher than the odds of pregnancy, depending upon whether the sex occurs during a fertile or non-fertile period and whether the person with the infection currently has symptoms.

For past Sex Question Friday posts, see here.

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