Dana McCourt pulls up some data. She writes:

[L]et’s look at the late-term abortions. Only 1.1% are after more than 21 weeks.  21 weeks is about two weeks shy of the lower-end of viability.  21 weeks is still in the second trimester.    We can safely assume that the number of abortions in the third trimester is even smaller, especially because abortion after 24 weeks is generally not permitted by law except in cases of danger to the health of the mother and the fetus.

... At 1.2 million abortions per year, 1.1% is about 13,200 abortions.  I mention this because late-term abortion looms much larger as a reason to oppose abortion than it should given its rarity.  It’s also important to note that past 24 weeks, the laws do not permit abortion that isn’t tied to reasons of life and health (fetal or maternal).   People primarily worried about late-term abortion need to understand what it is that they’re worried about, understand that what they’re worried about is already heavily regulated, and then need to make the case from there, based on the cases that actually occur.   It allows us to formulate the problem in terms of specific questions.  This doesn’t make the answers easy, or rid us of potential ethical questions.  But it at least ensures we’re not debating the abortion-equivalent of the ticking-time bomb.  To do otherwise is simply not based in reality.

(Chart from the Guttmacher Institute)