Citation? I can’t find anything to support this, just vague gesturing by organisations with no hard data. The only rigorous data I can find is a study from France which is irrelevant because France bans late-term abortions except for medical reasons. In fact I suspect that this is the cause of this belief, third trimester abortions are primarily medical, because most states in the US and countries in the world ban them except for health reasons. So of course the studies that address them are all going to be covering medically indicated abortions, and then journalists take this to the presses.

There is Kimport’s paper which doesn’t support your claim, but I find it quite shoddy regardless.