JENNIFER DERRICK’S weight had crept to 159 pounds from 125, and she knew she would not fit into her grandmother’s wedding dress.

“Women were smaller back then, and there was nothing to let out,” said Ms. Derrick, of Rockford, Ill. She took prescription pills, had vitamin B shots and made weekly $45 visits to a Medithin clinic in Janesville, Wis. When she married on March 18, she was back to 125 pounds; the gown, from 1938, fit perfectly.

In March, Jessica Schnaider, 41, of Surfside, Fla., was preparing to shop for a wedding gown by spending eight days on a feeding tube. The diet, under a doctor’s supervision, offered 800 calories a day while she went about her business, with a tube in her nose.

A 2007 Cornell University study by Lori Neighbors and Jeffery Sobal found that 70 percent of 272 engaged women said they wanted to lose weight, typically 20 pounds. So brides are increasingly going on crash diets, inspired by seeing celebrities like Sarah Jessica Parker or Gwyneth Paltrow, cowed by the prospect of wearing a revealing and expensive gown and knowing that wedding photos (if not the marriage) are forever.