If I remember correctly, rape related pregnancies are estimated at somewhere between 1 - 4.7% (Depending on the source you use), which is well below the consensual sex pregnancy rates. Anywho, I was able to find this on the internet without too much effort.
http://www.jfponline.com/Pages.asp?AID=4442#1
Quote:
Stress and fertility
Infertility—failure to conceive after 1 year of regular unprotected intercourse—affects approximately 10% of the reproductive-age U.S. population (Box).3-8 Does stress affect a woman’s chance of becoming pregnant? Research into this question—voiced by Mrs. S—has produced conflicting results.5,9,10
Stress does not universally prevent pregnancy; women have conceived as a result of rape. However, chronic extreme stress—such as that imposed by war, imprisonment, or starvation—can change the menstrual cycle. Effects range from subtle luteal-phase deficiency to menses cessation.9 It may be that evolution favored females of species who could “turn off ” fertility during stressful times to conserve physical resources and “turn it back on” and bear offspring after the threat passed.
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Therefore, I wouldn't instantly dismiss the OP's teacher as moronic, as there very well may be some truth to what (s)he said. Sure, she might have been trying to 'further her own agenda' or whatever you want to call it, but there's no denying that there is at least some information to back her claim up that stress tends to prevent pregnancies, albeit not to the point where it's 'nearly impossible'.