Women of the 18th century continued to wear stays (corsets) throughout the majority of pregnancy.
This may seem impractical and rather extreme our modern sensibilities, but stays were considered a basic and essential undergarment and part of everyday life. The suggestion of eliminating the use of stays during pregnancy would likely make no more sense to an 18th century woman than the suggestion of eliminating bras during pregnancy would to a 21st century woman.
Tailleur d’habits et tailleur de corps, a fascinating 1771 on tailoring (which can be viewed in its entirety here if you’re interested) has a design for a pair of stays patterned especially for pregnancy. These stays (shown above left) lace up the sides in addition to the traditional lacing up the back, allowing them to expand with a pregnant woman’s growing stomach.
The photo on the right shows what these stays would look like constructed.