
From experience with a singleton puppy, don't let her go too many days over or you will be looking at a C.Section, for one puppy!! Often they just sit, growing bigger by the day = more likely to need surgery!
The only problem we've had (and we've had singleton's twice, the first because one of the only two puppies was DOA) with singletons is they get very spoilt. I remember my true singleton 'litter', the bitch which we kept, was a real prima donna because once mum started spending more time away from her, and once being weaned especially, I used to carry her around with me rather than leave her all alone in the box. And her early experience showed! Thankfully the others showed her respect!