They probably did not know she was pregnant because her stomach may have seemed normal. If you look at most kids today they maybe thin, but have a big belly usually because:
*they all have organs inside their abdomen with no room to accommodate them, as they grow taller it all fits a bit better. When you see quite slim kids it is usually because they are quite active so their stomach muscles hold the organs in.

* A child of this age has a swayed back due to balance and walking and can make it look like their stomaches are large. This does improve as their spine develops more.

....I am surprised though.