In her excellent review of Abortion and Martyrdom, Tracey Rowland quotes two contributors as stating the Church would need to define the precise point of the infusion of the spiritual soul.
In his book, Redeemer in the Womb, John Saward leaves the reader with not the slightest doubt. He writes: "The rational soul, which is not transmitted by the parents, is infused by God as soon as the body is ready to receive it. It must be suitably organised. St Thomas explains how the human body is suitably disposed by the Divine Artist for those spiritual acts of which only man among bodily creatures is capable."
Saward then quotes L. T. Mangan, SJ: "The human zygote as we understand it today with DNA and RNA would in Thomas' understanding eminently satisfy as having the organised matter required for the infusion of a human spiritual soul." Sawara continues: "Thus man is a natural and complete whole from his conception. The embryo is alive with unmistakably human life. For St Thomas, that is just another way of saying that it is ensouled with a human (rational) soul. 'It' is already 'he' or 'she'."