According to the NRSV translation of Luke 1:41, Elizabeth’s “child leaped in her womb.”
The Greek here for “womb” (koilia) means “belly” or “stomach.” It’s the same word used of the snake in Genesis, for instance, which is punished to walk on its belly. Because snakes don’t have wombs, contexts like this show us that the Greek word is more general than “womb.” But “womb” is still a reasonable translation. And certainly we know that the “child” was in the womb, not some other part of Elizabeth’s anatomy, even if the original text was less clear.
The “child” here (brefos) probably refers literally to what we might now call an infant. But, like the (well accepted) shift from “stomach” to “womb,” I think we should translate this as “fetus.” Even if I’m wrong, though, I don’t think this word has much to do with the status of a fetus, for reasons I’ve already pointed out — in particular, the general way in which words are disconnected from the time at which they apply. (This is why the “child” in a woman’s womb in Leviticus similarly doesn’t tell us about the status of a fetus.)
What about the leaping? What was this child or infant or fetus doing?
The Greek is skirtao, and here we find a surprise. That Greek verb is used elsewhere almost entirely in two (related?) contexts: figuratively, and of fetuses.
In Genesis 25:22, Rebekka’s twin children “struggled together within her.” That’s skirtao.
In Psalm 114 (verses 4 and 6), mountains “skip”; in Wisdom 17:19, animals whose running is invisible “leap”; in Malachi 3:20, those who revere God’s name shall “leap” like calves; in Jeremiah, plunderers “frisk about” like a cow; and in Luke 6:23, God’s chosen should rejoice and “leap for joy.” All of those are skirtao in Greek. (Joel 1:17 uses the verb, too, but in a translation that doesn’t accord perfectly with the original text.)
So it looks like “leap” is only one possible translation, and probably not even the best. Perhaps “moved in the way that fetuses do” would be better. Or maybe “leaped for joy” in the same metaphoric sense of the English phrase, which indicates joy but not necessarily actual leaping.
One thing is certain, though. If we go with the NRSV translation of “leap,” we must understand the language figuratively. While fetuses can shift, kick, and otherwise move, actual physical leaping is beyond their ability.