I had a reply from my minister friend in Canada, and he had this to say:
"This is an interesting one. I have to say I've never run across this
sort of anomaly before now. I cannot say for sure or not whether there
would be any theological or superstitious issues involved here. But
I've never heard of such. What I can say is that I have never ever
heard of a case of a baptism being done pre-birth for any reason at all.
And I would have a hard time conceiving of one, especially in the 18th
century. The concept of life existing in the womb before birth is very
modern - 20th century. Even if they believed the child was at risk, I
cannot think that they might baptize pre-birth. Even today when we are
more aware of life pre-birth, in the event of a high risk birth I might
get called in to do an emergency baptism at the very moment of birth,
but never pre-birth.
So I would definitely favour this being a clerical error."
So thanks everyone, clerical error definitely seems to be the order of the day....!
Chris