As a FATAL Aedile, I can mathematically prove that rape decreases the chance of conception.
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A rape is a violent act and produces tremendous emotional and physical stress for the victim. Hormone levels can change drastically under severe stress, so it is possible that conception might not take place in the event of rape. Also, a woman would have to be ovulating when the rape took place for a pregnancy to happen, and this period of fertility is only a few days per month. Hence, the majority of the days of the month, a conception under any circumstances could not take place. That said, it is of course possible that a woman who is raped could indeed become pregnant.
While it is true that chronic stress can delay or halt ovulation, acute stress (like that of being raped) can induce ovulation and cause multiple eggs to ovulate at once. Whoever made that insenstive "legitimate rape" comment took the biology completely out of context, as again, it is chronic stress that can possibly halt ovulation.
This is the most ridiculous question. It's beyond me how someone who believes this was somehow elected to make decisions about anything. The egg isn't aware that the sperm is coming in against the victim's will. If any human had the power to prevent conception from happening, there would be no 16 and pregnant and certainly no need for a condom business.
I'm sorry to be so blunt, but the fact that this is a question shows an astounding lack of knowledge about basic human biology and the bodily functions and hormonal processes involved. Short, fact of the matter answer: no, unless you're lucky enough to be one of a specific breed of duck. Long answer: The human uterus doesn't care how semen gets there, it still does the same thing once it is. While it is true that rape, usually being a horrifyingly violent and terrifying act, would cause hormonal changes and a large amount of stress, this would not be sufficient to prevent fertilization or implantation of the embryo, although technically it may increase the chances of miscarriage later on if the stress levels stay high. The 'pregnancy means it's not really rape' thing was a through-the-teeth lie made up by reactionary fundamentalists who didn't have half a clue themselves and wanted more ammo to try banning abortion with.
Ducks literally have a system that prevents pregnancy if the female duck doesn't want it. Her vagina includes a decoy opening that the male's corkscrew-shaped penis gets directed into if the female does not wish to reproduce with him.
Humans on the other hand have no such mechanism to prevent rape-induced pregnancies.
Furthermore, anyone who says so clearly has no understanding of basic biology and human reproduction. The rape victim's reproductive system has no way of "knowing" that it's under attack. A woman who is capable of conceiving is going to be able to get pregnant just as easily from rape as from consensual sex.
It is not impossible for biology to prevent successful conception from rape, as one can see from the disturbingly fascinating biology of ducks. However, human bodies incorporate no such designs, and as such, any claim that a rape victim can prevent conception is fallacious at best. While some politicians have attempted to claim this, it is misinformation that is not backed by any serious scientific source.
Someone who is raped and inseminated will become pregnant. They have the same odds as someone who is having unprotected consensual sex. There is no way on earth a woman can prevent pregnancy with out medication or a condom. It would be ignorant to assume that one could prevent pregnancy during a rape.