I don't know how long the story is, but if it continues to the point where the character's pregnancy would show, she would need a maternity corset. This would be very similar to an ordinary corset, but with extra lacing on the hips, over the stomach, on the bust, etc. in order to gradually accommodate the changing figure without compressing the womb. (There were also maternity corset waists, which were generally made more lightly-boned and with straps, and buttoned in the front, sometimes with buttons on the hips for petticoats to attach to, as the weight of skirts on the waist was thought to be harmful.) These did not disguise the pregnancy.

Maternity clothes as such were not commercially available until 1904, with the opening of Lane Bryant. Prior to that, women would simply alter clothing to be looser or have looser clothing made up for them.

Prudence Saur's Maternity: A Book for Every Wife and Mother (1887) may be very useful to you for getting into what your character might understand about pregnancy.