The thing with trimesters is that there are still not clear lines between them. A woman can say "I better hurry up and get my abortion because this cluster of cells will be human tomorrow" but there's really no difference from one day to the other. Conception and birth are the only two instantaneous moments
Not to repeat myself, but what about viability outside of the womb? It's the same principle behind things like a legal driving age: does someone magically become able to drive a car at age 15 when they can't at 14? Of course not; but it's a good general indicator that doesn't require complicated (and expensive) testing.
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Houston Texans: 3-3
Division Ranking: 2nd