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jwalsh07 has already answered this question well, but here's my 2 cents:

Just as I cannot measure souls or another woman's faith, none of the early philosophers could measure a pregnancy until miscarriage or quickening. Although, it is possible that Pius IX was familiar with microscopic evidence of the early embryo.

However, in the case of the human embryo made in the petri dish, or whose existence can be followed by serial ultrasounds and his/her mother's serum HCG levels, we know better than any earlier thinkers could know. We know that there never is a time when the body is "unformed," only early stages of the human form. Those who are willing to endanger the lives of early humans can watch the cell division, down to the movement of the RNA and alignment of the centromeres and then the chromosomes.

We understand, as Robert George and Eric Cohen have written separately and together, and as has been seen and experienced in lab after lab all over the world regardless of the faith of the observer, that the body is living, and begins life at the penetration of the oocyte cell membrane by the sperm.

You make good points for those who believe in ensoulment to move it back further than did Augustine. But it is also important to recognize that he saw very early stages of development post-miscarriage, and it was that view of the early fetus that led him to feel that the human form was insufficiently developed at the early stages. Most during his time believed the opposite, but were certainly not in possession of any special knowledge to help them conclude that a soul existed at conception. He may have been inaccurate with respect to when the human form is sufficiently developed, but I suspect were it not for the Vatican, he would reach similar conclusions today.

He would better understand at what stage a central nervous system is sufficiently formed, when sex is determined, when pain can be "felt" and so on. But would he see a sufficiently formed human body a few days after conception? I doubt it.