"As one Catholic web site explains Be careful of equating one opinion with Church teaching, not only on this matter but when considering all questions relating to the Faith.
*Augustine believed that a male fetus obtained its soul at 40 days and a female fetus, between 80 to 90 days. Aquinas concluded that the soul was developed in three stages during gestation so that by the time the fetus was born he ir she was fully ensouled. Classical Judaism believes the soul originates in heaven and joins the body at body at birth, based on the Bible’s conclusion, found in Genesis, that after God formed man from the dust, God “breathed into his nostrils the breath of life; and man became a living thing.” *
Classical Judaism, Augustine and Aquinas did not have the knowledge of embryology that we have now and attempted to explain ensoulment and many other questions of the day with the understanding they had then.
The Catechism of the Catholic Church states that if a body is human, then it has a spiritual soul: “The human body… is a human body precisely because it is animated by a spiritual soul…” (No. 364). (Corpus hominis…est corpus humanum praecise quia anima spirituali animatur.)
When the sperm unites with the ovum a zygote is produced. the zygote possesses its own unique DNA; the zygote is human. Therefore the soul is present in the zygote.