It's actually fairly common for some women to gain very little weight, or if they are heavy to begin with, not notice an increase in weight. Also it's common for women to "spot" during pregnancy, and many take that as a normal period. It happened to a friend of my mothers, she thought she was having cramps and instead had a baby.

literal people are scary, man, literal people scare me
out there trying to rid the world of its poetry while getting it wrong fundamentally down at the church of "look,it sez right here, see!" Ani Difranco