Unfortunately, I think the answer to your question is that there is a large amount of scholarly disagreement on how early Christian leaders felt about abortion. Some scholars believe the early Christians took a nuanced stance, and some have written that they believed it was a sin at all stages. (But again there was no consensus on what degree of sin abortion would be)

There are some early Christians who believed that the embryo didn't have a soul at conception, and because of this early scholars were split on the issue of abortion.

In this situation we have a double edged sword, we have a wealth of information from several early Christian biblical scholars, but a wide variety of conflicting viewpoints.

It appears we are able to divide many early biblical scholars into a few camps:

Gregory of Nyssa, Maximus the Confessor- believed life began at conception

Didache, Clement of Alexandria, Tertullian, and saint Basil: condemned abortion without distinction.

Augustine of Hippo, Thomas Aquinas: both took influence from Aristotle's regarding "ensoulment," which basically means that the soul enters the fetus sometime after conception. And that after ensoulment abortion was homicide. However, it should be noted that even those who believed in ensoulment also condemned abortion after conception.

We have records of church councils (early medieval) punishing women for combined crimes (abortion + another sexual crime) as well as people who created drugs which induced a miscarriage. (I'm personally unsure if those "drugs," were universally effective)

----->

Early forms of punishment: The punishment for abortion (early/late) was usually the same in the early Catholic church.

Middle Ages: There was a heavier penance for later abortions, and a less severe one for abortion, "before life," (I personally took this to mean that before "ensoulment," there was a less severe punishment.)

We have records stating that the Catholic Church took its lead from Thomas Aquinas (and Aristotle by extension) and his views of successive souls (vegetative, animal, and rational) until 1869.

Post 1869 they removed the limitation of automatic excommunication to abortion of a formed fetus was removed.

We interpret that to be the Catholic Church saying: conception was the moment of ensoulment. (A bit of a neat combination of the two above camps)

Today: Things are even more split. In general the Catholics Church, Eastern Orthodox, Oriental Orthodoxy oppose abortion, as well as many evangelical denominations. However, this isn't the only viewpoint, and it's important to note that many Protestant, and Evangelical denominations believe in abortion with exceptions. Even a branch of Catholics (Catholics for Choice) have contrary views to the main body of the Church.

Sorry, unfortunately your question has a lot of varying answers. But I think the most cohesive one is that many early Church Leaders took influence from contemporary or historical influence- namely the Greeks, and this colored their interpretation of the issue.

If you're curious about more specific groups of thought, I'd really recommend the Oxford companion to Christian thought- it's quite comprehensive and very interesting.

Sources: Bakke, Odd Magne. When children became people: the birth of childhood in early Christianity.

"Abortion and Catholic Thought: The Little-Told History" (This is from the Way Back Machine)

Abortion and the Politics of Motherhood by Kristin Luker, University of California Press

The Oxford companion to Christian thought

M. Therese Lysaught, Joseph Kotva, Stephen E. Lammers, Allen Verhey, On Moral Medicine: Theological Perspectives on Medical Ethics (Wm. B. Eerdmans Publishing 2012