A number of different investigators have looked at this—Jonathan Abramowitz, in particular, and then me.
I focus specifically on harm thoughts. I have two publications on this. The first was about 100 people, and the second was about 400 people. The first time it was 49% who reported unwanted intrusive thoughts of hurting their babies on purpose. In the second iteration of somewhere between 400 and 700 people—I'd have to look at the exact numbers of who reported this—we had 54% report unwanted intrusive thoughts of hurting their babies on purpose.
We have, in both of those studies, looked at whether or not the people who reported these thoughts of hurting their babies on purpose were actually harming their infants, and whether or not that was happening more often in that group than in the group of people who did not report thoughts of hurting their babies on purpose. To date, we have found zero evidence, and the raw numbers would suggest that the people with those thoughts may—possibly, if we had a large enough sample—be slightly less likely to hurt their infants.