Is there an issue with ladies going to the zoo when they are pregnant? If yes then why is it a problem?
There is no issue with them going to a zoo, the problem is they should not be staring at the animals. I asked this question to HaRav Betzalel Rudinsky years ago and he showed me from Rishonim issues to this effect. I do not remember the sources, other than that the Raavad was among them.
The mekor is based off the Sefer Habris which explains it is not good for a pregnant woman to see strange things and then get scared which is bad for the baby. See here for answer #399 .
I see two mekoros; the sefer habris and the kav hayashar. I have been unsuccessful in finding any mekor in sifrei halacha and minhag (bar ilan didn't offer anything, but i'm not the greatest at searching...). I don't have access to these two great sefarim, but then again i understand that these sefarim say many things that we don't necessarily follow. I heard about this minhag from my mother and mother in law, and I would like to know if this minhag is the product of folklore so to speak, or it is actually endorsed by rabbanim.
Perhaps it is because of the snake who wanted to be intimate with Chava. The snake represents the animal kingdom... And because of the first sin of eating the fruit committed by Chava, she recieved the punishment of painful childbirth.