Well, scientifically, we know when the life of an individual human being begins. It begins at conception. You can only get life from life, Frankenstein to the contrary. A live child is conceived by living parents, and we say that its life "begins" because it is genetically a separate person from either of its parents.
The Talmud is presumably based on biblical descriptions such as the one in the Book of Job. But the Bible is not attempting to be scientifically precise at all points. The Bible says that the sun "rises" and "sets," but we know it doesn't actually do that--rather, the earth turns and it only seems to rise and set. But it's so convenient to say so that we still say that the sun rises and sets, which certainly is not a false statement. It merely has to be properly understood.
Similarly, St. Thomas and other theologians spoke about life beginning at the moment of quickening or ensoulment, taken to be when the baby is first felt to move in the womb. But we now know that life begins earlier, at the beginning, and that therefore ensoulment must also be earlier.
I must say, these days whenever I see the word "ethicist," I reach for my pistol (metaphorically). An ethicist is, more often than not, a highly trained professional who is almost completely ignorant of real morality. His function is to tell hospitals and other institutions who pay him what they want to hear.
St. Thomas and St. Augustine knew that life itself began at conception. They simply didn't believe it had advanced to the point of having a soul until it had developed to the point of movement. Nor was there any scientific evidence that changed the mind of the Church. It was "decreed" in the 1850s that life and soul began at conception. It was thought by some to be as much a political decision by the Pope as a theological one.
I know the feeling. "Medical ethics" has become pseudonym for creative ways in which to end life. This is the logical extension of abandonment of personal moral responsibility.