
Nobody can decide for someone else whether a new baby coming along is a good enough reason for rehoming a dog - everyone is different and everyone has their own individual set of circumstances. People can be far too judgemental.
Personally I have had a litter when my youngest (of five children!) was just under a year old and I coped but that was me. At the same time, I had very, very sickly pregnancies with all of my children and there were times during those pregnancies when I could have done without the dogs (and cleaning up after them lol) but soldiered on.
One has to assume that post natal depression may well play a part in the rehoming of some dogs when a new baby has come along and in that situation it is surely better that the new mum focuses on getting well - sadly it may be best for Mum and dog if it is carefully rehomed.