I was wondering how women (in the old times) were able to find if they were expecting twins. Was it possible a woman give birth to twins without knowing it?
Without ultrasound you’d discover by palpation - feeling the abdomen.
Each baby has two poles - the head and the bottom. If you can feel three clearly separate poles there must be at least two babies in there.
It’s difficult to feel 5 poles which would be needed to confirm triplets.
Alternatively, sometimes you can clearly identify poles which are heads - in which case the number of heads you can feel puts a lower bound on the number of babies.
This isn’t a particularly reliable approach - it’s easy to underestimate the number of babies. So multiple babies are often still a surprise if there is no access to imaging.