I will be surprised if one can find direct references to your question in scriptures. At least, there is nothing in the Vedas, Smritis and the Agamas.
So, it remains only to see if the PurAnas and the ItihAsas have something in them or not.
I am answering your question with an indirect reference, which says, a woman should be allowed to go to temples till the last month of her pregnancy.
From Bhairvai Tantram:
StrinAm Garbhavatinancha Naivya Dushanam Na KuryAddasame MAsi KritvA
Cha Naraki Bhavet |
There is no prohibition for pregnant women [getting initiated (taking
mantra dikshA)] but if taken on the tenth month the woman goes to hell.
So, if a woman is considered pure for the purpose of DikshA (which is the first and foremost among all rituals) for all months of pregnancy except the tenth, then she should be allowed to do any other religious activities like visiting temples, doing daily worship etc in those months.
But, of course, whether she feels like going to temples (because of her health condition) is another important thing to be taken into account.
NOTe- This is an indirect answer. So you should wait for more direct answers from scriptures.