TOKYO - Americans may be battling the bulge, but the Japanese are struggling to expand their waistlines.

The Health Ministry said the number of young, skinny women has risen to troubling levels. A record 29 percent of those in their 20s are underweight, according to a recent government survey. Those with a body mass index of less than 18.5 percent are considered underweight by Japanese standards.

"The women are not at risk of health problems yet, but we are making it a goal to bring the number down to 20 percent in the next decade," Yoko Saito, at the Health Ministry's Movement to Improve National Health, told ABC News.

The ministry has yet to come up with a plan to reduce the number of underweight citizens, but Saito said the government now treats diminishing waistlines as a national health problem, and worries that the problem could affect fertility rates. Japan already has one of the lowest birthrates in the world.

Twenty-four-year-old Shiho Aoki said the pressure for young women to be thin is greater than ever, largely because popular Japanese models are getting skinnier. At 5 feet 1 and weighing 100 pounds, Aoki said she was considered "fat" among friends.

"I had a child 8 months ago, but I've been struggling to lose weight ever since," Aoki told ABC News. "I'm very jealous of my friends who are skinny."

Nineteen-year-old Mika Asai said her friends skip at least one meal a day, to cut back on calorie intake. She's refused to follow suit, so far

"I love food too much to diet," Asai told ABC News, though she's hardly "fat" at 5 feet 2 and 110 pounds. "I'd rather be healthy."

Japan is already among the thinnest industrialized nations, thanks to a diet of fish, vegetables - and small portions. But while young women have become too skinny, Japanese men have moved to the opposite side of the spectrum. Nearly 40 percent of men in their 40s and 50s are considered overweight (their BMI is greater than 25 percent, about 20 percent higher than women the same age).

Weight gains prompted the government to impose waistline standards four years ago. Companies and local governments are now required to measure the waistlines of men and women between the ages of 40-74 as part of their annual health checkup. Employees who exceed the standard - 33.5 inches for men and 35.4 inches for women - are asked to undergo counseling . If they still fail to slim down, their companies face government fines.