He is studying exactly this. He found that when developing, cells move and grow in a particular direction. But when an obstacle is placed in their way, they move around it to somehow still end up where they’re supposed to.
DNA is not all of the encoding. Even cells appear to have some form of navigation or space search capabilities.
In addition to those unavoidable ones, there can also be things going wrong by accident (e.g. the pregnant woman suffer physical injury which kills the fetus).
for example (pre)eclampsia. we still don't know WTF is going on exactly (but it's basically abnormal blood vessel formation between the fetus and the uterus), and a few decades ago it was basically guaranteed loss of the fetus or the mother (or both), in about half a percent of pregnancies.
nowadays thanks to medical science it's a hundred-times more manageable.