How do we know what religion women getting abortions identify as?

There was just a discussion about abortion and people were giving statistics about the religion of women getting the abortions- they said something like 70% were theists, and some others had some other stats. I have had a couple of abortions (regrettably, but true) and never wrote down my religious affiliation. In fact, it's not even legal to include that info in medical records unless it's for refusal of treatment due to religious standards. So where are they getting these stats? I'm not agreeing or disagreeing, but just curious where this info comes from- I was never asked. Who was?