As soon as Keiko Ozaki found out she was pregnant with her second baby, she went on a diet.

Ozaki, who teaches jazz dancing in Osaka and weighed 51 kg, said she was chastised by her doctor for gaining weight too quickly during her first pregnancy. With her second baby, Ozaki, 30, avoided rice and skipped meals before her monthly checkups, adding 7.8 kg over nine months — 4.7 kg less than U.S. women gain on average.

She managed to avoid her doctor’s rebukes, but her baby boy was 300 grams lighter than the national average of 3 kg. As Japan’s birth weights fall for a third decade, scientists say advice that pregnant women receive may be contributing to the highest rate of low-birth-weight babies in the developed world. More critically, it may be setting their infants up for diabetes and heart disease later in life.

“What’s happening in Japan is a shocking phenomenon,” said Hideoki Fukuoka, who studies evolutionary biology at Waseda University in Tokyo. “You hear of doctors yelling at expectant mothers and telling them to transfer to another hospital if they can’t manage their weight.”

The national society of obstetricians plans to follow the U.S. and U.K. in publishing guidance on weight-management in pregnancy in April. The U.S. Institute of Medicine’s guidelines were released in May 2009 and Britain’s National Institute for Health and Clinical Excellence issued its latest advice in July.

Unlike most developed nations, where new moms are getting heavier, in Japan they’re becoming thinner. The result is that the average weight of a newborn in Japan is 200 grams less than in 1980. The prevalence of babies weighing less than 2.5 kg — low birth weight by World Health Organization standards — is now 9.6 percent, up from 5.2 percent three decades ago.

A baby’s low weight at birth is either the result of preterm delivery or restricted growth inside the womb, according to the WHO. It says birth weight is affected by the mother’s own fetal growth and her diet from birth to pregnancy, and her body composition at conception.

Newborns weighing less than 2.5 kg are at greater risk of dying and suffering poor health and disability, WHO says. A growing body of scientific evidence also shows growth-restricted infants are at increased risk of coronary heart disease, high blood pressure, stroke, Type 2 diabetes and obesity as adults.

“The nine months during pregnancy and the first two years of life will set the foundation for good health for the entire life course,” Margaret Chan, the WHO’s director general, said.

Another consequence of undernourishment is an increasing rate of spina bifida due to inadequate intake of folic acid, which the body uses to make new cells, according to Waseda University’s Fukuoka. Women lacking the B vitamin weeks before and around conception are at greater risk of birth defects caused by the incomplete development of the spinal cord or its coverings.

A 2007 health ministry-funded survey by Yokohama City University found 4.8 Japanese babies per 10,000 were born with spina bifida, compared with fewer than 1 case per 10,000 in the mid-1970s. In the U.S., where cereal grains have been fortified with folic acid since 1998, the prevalence of spina bifida was 1.9 per 10,000 live births in 2005, according to the U.S. Centers for Disease Control and Prevention.

The growing fetus adjusts to environmental cues and adapts its development to match, according to Peter Gluckman, New Zealand’s chief science adviser, who has studied human development and disease for 30 years. Gluckman’s team at the Liggins Institute in Auckland found that adaptive changes may increase the risk of diabetes and heart disease if the fetus predicts a life course of poor nutrition and encounters an excess outside the womb, leading to a mismatch.

“Being born small and growing big is the worst possible scenario for risk of disease, at least from the mismatch side of things,” said Mark Hanson, chairman of the International Society for Developmental Origins of Health and Disease and a collaborator of Gluckman’s at the U.K.’s University of Southampton.

The incidence in Japanese children of Type 2 diabetes, the variety in which the hormone insulin is unable to regulate sugar in the blood, almost doubled from 7.3 per 100,000 from 1976 to 1980 to 13.9 per 100,000 in 1991 to 1995. Still, it’s too early to tell whether the decline in birth weights in Japan has made the nation’s citizens sicker in adulthood, Hanson said.

“Japan is an extraordinary country,” he said. “It’s the only developed country in the world which is increasing the risk of its disease by both reducing the adequacy of nutrition of the fetus at one end and then Westernizing at the other end.”

More than 22.5 percent of women in their 20s are underweight, having a body mass index of less than 18.5, the health ministry said last year. Among women in their 30s, 16.8 percent were underweight. Normal-weight people have a BMI of 18.5 to 24.9.

“Many women, especially in cities in Japan, are now espousing very much the kind of body image that you associate with the magazine model: the stick-insect-thin model,” said Hanson, who has studied birth weight trends in Japan.

A rise in smoking among young women in Japan may also be contributing, the Organization for Economic Cooperation and Development in Paris said in a report last year.

Being slender makes it more difficult to tell when a woman is pregnant, said Mami Nomura, who had a BMI of 16.4 before gaining 10 kg during her pregnancy this year. The Tokyo mom routinely wore a tag given out at subways that said “I’m pregnant” to encourage passengers to give up their seats for her on buses and trains.

“Foreign friends couldn’t tell I was pregnant and would ask me, ‘where are you hiding the baby?’ ” said Nomura, 38, who delivered a 3.2-kg infant in July.

In a country where fewer than 1 in 10 maternity hospitals can offer women an epidural, the anesthetic procedure that numbs the lower half of the body, smaller babies may be easier to deliver.

Osaka obstetrician Hiratoshi Sakamoto, who delivered Ozaki’s babies, said he has told patients to manage their weight for 25 years after seeing heavier women struggle through difficult labors with larger babies, putting the infants at risk.

“No matter what I tell them, 80 percent of normal-size women end up gaining 13 to 15 kg,” Sakamoto said. “Babies can comfortably be born if they’re around 2.6 kg.”

In 2006, in reaction to the increase in low-weight babies, the health ministry said normal-weight women should gain 7 to 12 kg during pregnancy, less than the 12 to 17 kg recommended in the United States. Many doctors in Japan still are reluctant to recommend that pregnant women put on enough weight, according to Waseda University’s Fukuoka.

“Weight control has been the most important agenda item for them and they’re reluctant to accept that what they’ve been practicing for decades may turn out to be wrong,” he said.

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