Extreme stress reduces the chances of not only conception, but also viable pregnancy. Early miscarriage is very common even under the best of circumstances. It is not as though birth is inevitable whenever conception occurs. It is actually more likely it will be expelled. And even more so when under extreme stress.
Some women who are raped will still become pregnant and maintain the pregnancy. But it is less likely. If they do, it does not make their rape any less "legitimate." But it is scientifically true that it is less likely.