A - Thanks for the question. St. Thomas isn't the first or the last to teach that ensoulment doesn't happen for a certain amount of time after conception, usually cited as 40-80 days. The common scientific belief of ensoulment was based on the science at the time. The problem with the science is that it relied on Aristotelean biology. This was a belief that after intercourse the semen would mix with the woman's blood and then cause fertilization. This would then proceed through several steps of formation including the vegatative state - animal state - to the rational state. Then the body was properly "prepared" to receive the soul.
So, the problem was that the science at the time wasn't properly disposed to understand human formation and therefore the philosophy of the beginning of life also lagged behind.
We now know the stages of fertilization, implantation, and growth of a baby. At all stages, from conception until death, there is nothing else the life can be except a human being. So, St. Thomasn (and St. Augustine among others) didn't have enough information to make an informed decision about life because their scientific knowledge was limited and incorrect. So, when others cite their teachings about this issue, it is done to do one of three things:
- Distract from the real issue at hand, or
- To try and show the Church has "changed doctrine" and therefore is untrustworthy, or
- Because of their own ignorance.
None are based in scientific reality.
I hope this helps.