I have no idea - I'm involved in a local branch of Sands and there are a number of research studies ongoing into antenatal care particularly in the third trimester so hopefully they will shed some light.
In my daughters case, there was no real reason (acute oxygen loss but no explanation), despite additional consultant led care in the third trimester for an unrelated risk. So while I sadly know many families whose children could have survived of their care had been better, I'm one of the unknowns and I swing from finding it easier and harder that this was the case. It is unbearably cruel and sad regardless.