In Early Pregnancy
In the beginning of pregnancy the uterus is completely protected by the pelvis, so a miscarriage caused by being hit in the stomach is not likely.
The bones of the pelvis act as a protective barrier. The uterus isn’t exposed at this point, so you can breathe a sign of relief that falls nor trauma are likely to be problematic.
In the Second and Third Trimesters
It’s only after the first trimester, when the uterus begins to peek above the pelvis, that there’s more likelihood that damage may be done. The risk here is that severe trauma would cause a placenta abruption. However, note that this type of damage is typically not caused by your average fall or minor car accident. Also remember that your baby is protected by the amniotic fluid, so damage is more likely to come to the placenta more than anything else.