Well IMHO, the OP is asking advice on what to tell his child. I would think he should tell that it can be an abortifacient. It may not be “proof”, but the following is good enough proof for me. And why would a Catholic parent leave out the possibility of the pill being an abortifacient just because someone can’t say 100% that it’s correct? Why would a catholic take the chance and say well we don’t know it’s an abortifacient so we’ll pretend it’s not when the evidence says otherwise:
aaplog.org/collition.htm
"On March 24, 1997, I had a lengthy and enlightening talk with Richard Hill, a pharmacist who works for Ortho-McNeil’s product information department. (Ortho-McNeil is one of the largest Pill Manufacturers.) I took detailed notes.
"Hill was unguarded, helpful and straightforward. He never asked me about my religious views or my beliefs about abortion. He did not couch his language to give me an answer I wanted to hear. …
"I asked him, ‘Does the Pill sometimes fail to prevent ovulation?’ He said ‘yes’. I asked, ‘What happens then?’ He said, ‘The cervical mucus slows down the sperm. And if that doesn’t work, if you end up with a fertilized egg, it won’t implant and grow because of the less hospitable endometrium.’ (Emphasis in the original)
"I then asked Hill if he was certain the pill made implantation less likely. ‘Oh yes,’ he replied. I said, ‘ So you don’t think this is just a theoretical effect of the Pill?’ He said the following, which I draw directly from my extensive notes of our conversation.
“Oh, no, it’s not theoretical.
It’s observable. We know what an endometrium looks like when it’s rich and
most receptive to the fertilized egg.
When the woman is taking the Pill, you can clearly see the difference, based both on gross appearance - as seen with the naked eye - and under a microscope. At the time when the endometrium would normally accept a fertilized egg, if a woman is taking the Pill
it is much less likely to do so." (Emphasis in the original)”