It's worse than that. People get to interfere in mice. You can stunt their growth, give them transparent skin, grow more or less limbs, cut into them ... you can't experiment at all on humans.

Especially when it comes to pregnancies we know more about a lot of animals than about humans. Why? Well pregnancies is how you multiply meat in animals, which is what farmers are interested in (and pay for). Which ironically also means animal pregnancies can be treated in case of trouble much more effectively.

Why pregnancies? Pregnancy changes a LOT of chemical processes in the body and so quite a bit of "normal" medical knowledge doesn't apply to pregnant women. Which has caused the medical establishment to declare anything that isn't explicitly tested on pregnant women as a no-go zone. So even problems and medications that we do know about, doctors won't apply them to pregnant women.

Yes there are metabolic changes in the mother herself during pregnancy but that's not why it's hard to research. The main fear is that drugs will cross the placenta and affect the growing fetus, or similarly be transmitted through breast milk to an infant. Very young humans are uniquely vulnerable to disruption in their growth that can cause life-long problems.