The
Catholic Church does not teach that the human
soul is contained in the
ovum. Furthermore, the official Catholic opposition to abortion and its opposition
in vitro fertilization are based on totally different principles.
Catholic opposition to abortion is based on the assumption that human life begins at conception. From a scientific point of view this is an entirely reasonable position--we now know that the basic "nature" of a human being is created as soon as a particular egg and sperm cell have united to form a zygote. The DNA in that single cell doesn't change, and so it's pretty reasonable to say that the single cell is human.
Thomas Aquinas believed that the embryo formed gradually over time, and that it wasn't recognizably human until a certain point in its development. The embryological beliefs of Thomas Aquinas aren't a particularly interesting topic, but it's sufficient to say that he had highly mistaken ideas about human development. His treatment of abortion is reasonable--if his beliefs are correct--but they're not.
The Catholic opposition to birth control and artificial insemination is based on a very different issue; Catholic teaching says that sex exists for two equally important purposes--the continuation of the human species and the union of two people. Pope Paul VI, in 1968, presented as the official Catholic position that every single act of sex and procreation must be linked--thus, it isn't legitimate ever to use artificial forms of birth control, or to procreate without an act of sexual intercourse.
According to the Catholic belief, 40-50% of the human race dies within days of their conception. This is something that Catholic theologians should be thinking about more than they do. However, Catholicism does not teach that these unborn and unbaptized embryos are stuck in Limbo. Limbo was never part of the official teaching of the Catholic Church (although it was taught in most American Catholic schools until the 70s). Furthermore, the Catholic Church doesn't teach that a person has to be baptized to be accepted by God; in fact, a Boston priest was excommunicated in 1943 for saying that only baptized Catholics could be saved. So that argument is also not really valid.