There are a number of factors.
A woman is only fertile for about two days a month and sperm can live for up to three days. So, there are maybe five days a month that a woman can get pregnant.
However, this does not factor in the reality that many women are on birth control and many rapists use condoms, this alone greatly reduces the number of pregnancies. Then there is the fact that the trauma of the rape can often be stressful enough to prevent fertilization. Add to this the fact that many rape victims are menopausal women.
Everything I've read indicates that there are between o5 and 95 thousand rapes per year in the United States and the general consensus is that no more than 5% of rape victims become pregnant. So, the actual number is fairly low.
Nevertheless, the problem with Akin is that he stated that there is some sort of biological defense that keeps rape victims from becoming pregnant and this is flat out false. Add to that his use of the term "legitimate" was stupid.
This SHOULD read between 80 and 95 thousand.