Sam, I asked my husband and he says that in Hilchos Megilah O.C.686, S. Beis, the Rama says that on the ones who are mitzta’arim harbeh, afilu ko’avei eneim, don’t fast, v’yifre’u achar kach. So they’re supposed to mashlim it later. He told me that it is his chiddush, so I am not going to argue his position since I don’t completely understand all of the details.
42 – My husband says that Ra’avid holds that even pregnant women should fast but there are lots of poskim that don’t hold like this. Perhaps the women that BPT is referring to have their minhag based on the Ra’avid.