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.

In the two months of fittings before most clients’ weddings at Kleinfeld Bridal in New York, seamstresses are kept busy taking in gowns. Brides-to-be say, “I don’t want the size 16, I want the 14 or the 12,’ ” said Jennette Kruszka, Kleinfeld’s marketing director. (Note: wedding dresses run small, so a typical size 8 dress will be a size 10 or 12, a psychological shock for the young woman buying the gown. Kleinfeld resists brides who want smaller sizes, because taking a dress in is easier than letting it out.)

“There was no way I could imagine seeing myself in a strapless bridal gown,” said Colleen McGowan, 36, a New Yorker who used Weight Watchers and hired a personal trainer to drop 20 pounds for her wedding nine years ago. Her trainer, Sue Fleming, a fitness coach in Brooklyn, is the author of the book “Buff Brides.”

“I’ve been training brides for 12 to 13 years, and the typical weight loss is 15 to 20 pounds,” said Ms. Fleming, who charges $140 to $200 a session. One of her customers was so proud of her improved arms that she dropped and did push-ups before walking the aisle.

Brooke Baxter, 34, a music entrepreneur in Brooklyn who got engaged in November, ordered the BluePrintCleanse, which costs $65 a day for six bottles of organic juice. It helped her drop six pounds in three days. She will cleanse again before her wedding in May, she said.

BluePrint actively courts brides, even suggesting on its Web site “a cleanse with your whole bridal party.” Erica Huss, a founder of BluePrint, explained the effect of the diet as “You’re working out all the bad stuff.” Put another way, an all-juice diet is a laxative.

But the many “detoxing” cleanses make misleading claims, says Dr. David Gorski, an associate professor of surgery at Wayne State University in Detroit and a blogger at Science-Based Medicine. “Do you notice they never tell you what the toxins actually are?” he said. “There’s no science to back them up.”

Fad diets are cyclical. Master Cleanse (lemon juice, paprika, a dash of maple syrup, all in water, for 10 days) was published in a 1976 book and returned to popular consciousness with Ashton Kutcher and Demi Moore two years ago.

“Celebrities are not known for doing things that are necessarily the healthiest or most sensible in many parts of their lives,” said Dr. Louis Aronne, the director of the Comprehensive Weight Control Program at NewYork-Presbyterian/Weill Cornell Medical Center. “Nutrition should probably be included in that.”

The Atkins diet inspired the South Beach diet, which, most recently, has been popular as the high-protein, low-fat Dukan diet. “I didn’t want to be a fat bride,” said Casey Crisefi, 31, of Hagerstown, Pa., who lost 70 pounds on Dukan.

One of the most drastic diets involves daily injections of human chorionic gonadotropin, a hormone associated with pregnancy. The method was popularized in 1954 when Dr. Albert T. W. Simeons published “Pounds and Inches.” The fad is back, and in December 2010, the Food and Drug Administration reiterated its findings: over-the-counter human chorionic gonadotropin was fraudulent for weight loss and illegal, and a prescription for it does not help weight loss.

Dr. Cynthia Macer of the MediLean clinic in Rolling Hills Estates, Calif., says she tells her patients “flat out what the F.D.A says.” But they lose weight by combining the injections, weekly meetings with a registered nurse and a 500-calorie diet. The $950 cost includes medical screening, the hormone and instructions in self-administering the shots.

Lindsay Gardner, 28, lost 14 pounds on the diet in six weeks before her wedding last year, but critics insist the injections are a placebo.

Something medical is indeed happening in the newest diet to reach the United States. Dr. Oliver R. Di Pietro has been offering what he calls a K-E diet at his modest clinic in Bay Harbor Islands, Fla., since last July.

“I get a lot of brides,” Dr. Di Pietro said. “Nervous eating.”

It uses a nasogastric tube (a tube that goes through the nose and down the esophagus into the stomach) to provide all nourishment, with no carbohydrates, for 10 days. Dr. Di Pietro said body weight is lost quickly through ketosis, the state in which the body burns fat rather than sugar. Patients at his office are monitored during the 10-day period for things like constipation, bad breath and dizziness.