Raped women pregnancy rate. Do they shut the whole thing down?

Missouri Congressman Todd Akin’s infamous comment about women being able to avoid pregnancy in a “legitimate” rape and the subsequent media frenzy made me wonder if there was any truth in it, if you think about it, the stress and depression a rape generates could be so extreme that women’s body might after all “shut the whole thing down”. So I’ve estimated the chances for this to happen and it turns out that this politician was… oh, surprise.

400px-RapeSabine

After the Akin’s foot in mouth we could read all over the Internet media quoting gynecologists saying that chances of pregnancy are the same regardless whether a woman is raped or not. So people’s snarly comments and the “this is obvious, you gotta be stupid to believe otherwise” attitude we observe when anything gains political momentum spread like wildfire. It is at this point when questioning the crowd makes us feel uncomfortable but, since I know how much trust and credit I can give to media, I overcame my lack of comfort by running the numbers myself, and well, turns out that Mr. Akin might be as right as wrong since up to this day we truly have no data nor studies to unequivocally suggest that  women shut the whole thing down or open the whole thing up when raped.

The Numbers

In order to figure out whether Mr. Akin is right or not we need to compare the probability of pregnancy per intercourse in a random day within a menstrual cycle when the relationship is consensual with the probability when the intercourse is forced.

Consensual Pregnancy

A study from 2002 carried out among 782 healthy couples investigating changes with age in the level and duration of fertility in the menstrual cycle yield the following results:

Probability of Pregnancy Following IntercourseBeing zero the ovulation day, women have around 11 fertile days in their cycle (though only 5 of them with chances noticeably above zero) and the most fertile moment takes place two days prior to ovulation, where young women have around 50% chances of pregnancy.

But to calculate the probability of pregnancy by having intercourse just once in a random day within a menstrual cycle we need to know as well how long the cycles are. Using the average length of 28 days might give us a good estimation but to increase accuracy we can use the distribution of the cycle’s length measured in days. According to William Obstetrics, 23ed the ovarian cycle behaves in the following fashion:

Incidence of Menstrual Cycles

Once we consider both studies we can estimate for every range of ages the probability of pregnancy per intercourse in a random day within a cycle. The estimations are shown in the following table:

Pregnancy probabilities (one intercourse in  a random day within the menstrual cycle)
Age 19-26 27-29 30-34 35-39
6.00 % 4.82 % 4.79 % 3.82 %

Forced Pregnancy

According to the U.S. Bureau of Justice around 80% of rapes occurred to women under the age of 30. That means that we should focus in the 19-26 and the 27-29 ranges in the previous study for comparisons and, consequently, if rape has no biological effect in a pregnancy, the probability of pregnancy after raping a random young woman in a random day should be somewhere between 4.82% and 6.00%.

There is a commonly cited study about rape-related pregnancies which claims a rate of pregnancies of 5% per rape among victims of reproductive age. Reading this study one might think that:

“Generally, a single act of a rape has about a 5% chance of resulting in pregnancy among victims aged 12 to 45 who are not on birth control…” (CNN)

But it is not clear that the paper equates this rate to the probability of getting pregnant after one instance of rape. I myself could only read the summary of the paper since it is not freely available, but bloggers at Scientific American like Kate Clancy had the chance to study it and it seems the paper has problems in its numbers. A excerpt I have from Kate’s article states:

…A total of 30 women [from 616 instances of rape noted] reported one rape-related pregnancy and two additional women reported two rape-related pregnancies. Of the 34 cases…

But I don’t know if those 616 women were already using contraceptives at the time they were raped or even if they were asked in the interview if there was a chance they were already pregnant.

Anyway, if we assume all 616 instances occurred to women using no contraceptives and that they did not engage in unprotected sex since last menstrual cycle before the rape, then the rape-related pregnancy rate would be 34/616 ~ 5.5%. So where does the 5% comes from? There are studies that even suggest that the 5% figure should be corrected to a 2% since  more studies suggest that 60% of women who claimed pregnancy after rape were actually impregnated by a consensual mate,

But, if the 2% figure is near to correct, this would give Mr. Todd Akin munition to support his point! Nonetheless, we could still explain lower figures arguing that ovulating women avoid risks and, therefore, this reduces the percentage or rape related pregnancies, or that rapist do not always ejaculate inside the woman if at all.

So since this source was not being very helpful, or even trustworthy,I kept looking for more studies to see if there was any that was unarguably convincing; I didn’t find it, and if all these previous studies were not already making the situation confusing enough I found studies like this one  which estimates the probability of pregnancy after rape in a whooping 17%! Which means that Mr. Todd Akin would be wrong but in a totally unexpected way! Do women actually open the whole thing up when raped? Once again, we don’t have to believe this is due to biological reasons but maybe due to false rape reports to avoid social pressure or other social circumstances.

Well, there you go, choose the truth you like the most and pick your study accordingly, I myself at this point have no clue whatsoever whether women have ways to shut the whole thing down or open the whole thing up, and yet, everyone around me seems to be so certain… oh well.

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