What gave you that impression? Something you read by St. Thomas or St. Francis?
There has been a question of when ensoulment of the baby in the womb occurs. This is not the same as validating abortion even prior to the putative ensoulment.
Certainly either saint is entitled to their opinion; that wouldn’t make it the opinion of the Church. However, I would like to know what is the source of your impression.
and as I reflect while writing this I am reminded that the devil rattles the brush to distract one from the forest. Or something like that. That is, the debate about when a life begins is just a distraction which keeps people tied up while more and more lives are ended and ruined.
You make a great point -- thank you.