I, too, understand without having the exact reference, that pregnant women are exempt from fasting and abstinence.
However, if that were not the case, I think it’s quite possible to observe what the Church requires without endangering either the mother’s or the baby’s health.
There are only two fast days - Ash Wednesday and Good Friday. On these days, one may have one full meal and two small meals, all meatless. There is no restriction on beverages, including nutritious, protein-rich milk.
On days of abstinence, it is not required to abstain from protein foods, but from the flesh of warm-blooded animals and birds. That leaves plenty of protein-rich foods still on the menu - fish, eggs, cheese, beans, soy products, peanut butter, milk…
Could it be that many doctors are not Catholic and still have in mind the more rigorous fasting of the past when they make their recommendations?
Betsy