I've read a great book about Life between Lives which touches on this. In it an old soul explains that the souls weaves in and around the physical form of the developing baby but does not fully meld with the physical form for a few years after birth. The soul sometimes leaves the body and plays on an astral plane. They use this to explain why sometimes babies die - their soul is playing and they forget to come back to their body. I hope this idea doesn't upset anyone who has had a baby die from SIDs

The other thing they mentioned is when children die young, their incarnation was not for their soul's benefit, but for their parent's. I can accept this on an intellectual level but struggle emotionally. The thought that our son died because of our karma is very distressing. On the one hand, I know he did not suffer and we were truly blessed to have had him. On the other, I feel guilty that something I've done in a previous life has caused this. If I chose this before I was born, I really hope there's a damn good reason for it. It's something I find hard to deal with.

They also explain that there is no judgement on termination. It does not harm the baby's soul. The same with euthanasia - there is no soul benefit for pointless physical suffering - so ending that suffering does not affect the soul's evolution.