I suspect that was smallest baby the baby snatcher could find. :)
Must be difficult to plan these things, though. Get a few pregnant women to agree to their baby being used for filming and hope at least one gives birth to a healthy baby at due date, and the mother doesn't change her mind.
They should have contacted Call the Midwife's producers for how to do it, and maybe borrow one of their prosthetic babies. Also, as I've said before: who cut the cord?!