
Clothing was often returned to its original form after it was altered as it was more economic to do so. Developing an eye for the signs of changed garments - pin holes or mended seams around the waist - can provide a clue as to how women dressed while pregnant. Also, taking a better look at women who appear conspicuous in photos might lead us to further understand women's experience while being pregnant.
There are few sources available on maternity wear in this period as there is little material evidence of women being pregnant. Books like A Cultural History of Pregnancy:Pregnancy, Medicine and Culture, 1750-2000 by Clare Hanson and Bound to Please: A history of the Victorian Corset by Leigh Summers explore this fascinating subject but there has been no comprehensive study specifically on maternity wear (please correct me if I am wrong!). A short article by Zuzanna Shonfield, "The Expectant Victorian, (Late 19th Century Maternity Clothes)" (Costume 6 (1972): 36-38) is also an excellent guide to the cultural context surrounding maternity wear during this period.
Beautiful b&w image by Tom and Katrien found on Wiki Commons.