“The image Japanese mothers-to-be are striving for is the look of having a basketball in front of them while the rest of the body is slim,” says Naho Morisaki of Japan’s National Center for Child Health and Development in Tokyo.

Kim Kyung-Hoon/REUTERS

Staying slim during pregnancy carries a price

Japan's obsession with slender women may harm unborn children and create long-term health problems for the Japanese population. Already, a high proportion of Japanese women is starting pregnancy underweight, and many scientists have criticized the country's official guidelines for weight gain during pregnancy as too strict. Now, a survey shows many pregnant women strive to keep their weight gain below even those targets. This combination of factors has led to an unusually high percentage of low-weight births, which is likely the reason that the height of the average Japanese adult has declined every year for those born after 1980.

The impact could go far beyond height, says perinatal epidemiologist Naho Morisaki of Japan's National Center for Child Health and Development in Tokyo, who led the new study. "Japan may experience an increased disease burden among adults, and there could be an impact on longevity," she says. People born small are more prone to diabetes and hypertension, says Peter Gluckman, an expert on the developmental origins of health and disease at The University of Auckland in New Zealand, who calls the situation "really alarming." "We've tried very hard to convince Japan's authorities" to revise the weight gain recommendations, Gluckman adds. But a spokesperson for Japan's Ministry of Health, Labour and Welfare says there are no plans to do so.

The shortening of the Japanese is subtle, but unmistakable. An international study published in 2016 found that since the late 19th century, the average Japanese adult male height rose 14.5 centimeters, peaking at 171.5 centimeters for those born in 1978 and 1979. But by the 1996 birth cohort, it had dropped to 170.8 centimeters. Over the same period, average female height jumped 16 centimeters, topped out at 158.5 centimeters, then dropped by 0.2 centimeters. Some other countries have also experienced height declines, which the study variously linked to economic privation, an influx of shorter immigrants, or—in the United States—poor diet quality, which can impair growth both in the fetus and in newborn babies.

In Japan, experts say the evidence for a link with lower birth weights is strong. As the country recovered from World War II, the percentage of low–birth weight babies—those weighing 2.5 kilograms or less at delivery—declined from 7.3% in 1951 to 5.5% in 1978–79. As babies grew heavier, however, doctors worried about preeclampsia, a complication that can put the lives of both mother and baby at risk. In the late 1970s, some Japanese obstetricians suggested a low-calorie diet could lower that risk, a view incorporated into 1981 guidelines from the Japan Society of Obstetrics and Gynecology. "Previously, mothers-to-be were told to ‘eat for two’; now, the ideal is to ‘give birth small but raise a big baby,’" says Hideoki Fukuoka, an obstetrician at Waseda University in Tokyo.

Honey, I shrunk the population

The frequency of low birth weight in Japan started to rise after 1980; average adult height for people born in the years since then has declined.

N. MORISAKI, ET AL., JOURNAL OF EPIDEMIOLOGY AND COMMUNITY HEALTH 10.1136/JECH-2017-209266 (2017), ADAPTED BY J. YOU/SCIENCE

Health ministry recommendations issued in 1995 also reflected the concerns. The ministry adapted guidelines for U.S. women, produced by what was then called the U.S. Institute of Medicine (IOM), to the smaller and lighter Japanese population, but in doing so made them considerably stricter. For underweight women—those with a body mass index (BMI) below 18.5—IOM guidelines suggest a weight gain of 12.7 to 18.1 kilograms; Japan set the range at nine to 12 kilograms.

Japanese women took the advice to heart, and the percentage of low-weight babies rose to 9.6% in 2010. That this caused the drop in adult height "is entirely credible and fits with what we know from [research into] third world nutrition," Gluckman says. Morisaki has now confirmed that the desire to stay slim is exacerbating the trend. Today, more than 20% of Japanese women in their 20s have BMIs of less than 18.5, compared with 1.9% of U.S. men and women aged 20 to 39. In a survey of 1681 pregnant women, 54% said their ideal gestational weight gain was below the recommendations, Morisaki's team reports in a paper scheduled to appear online this week in Scientific Reports.

N. MORISAKI, ET AL., JOURNAL OF EPIDEMIOLOGY AND COMMUNITY HEALTH 10.1136/JECH-2017-209266 (2017), ADAPTED BY J. YOU/SCIENCE

"The image Japanese mothers-to-be are striving for is the look of having a basketball in front of them while the rest of the body is slim," she says. The survey found that in addition to a quicker recovery of their prepregnancy figures, women hope for easier pregnancies and fewer birth complications. But follow-up surveys found that lower weight gain did not reduce the risk of cesarian delivery or lead to faster postpartum weight reduction. And the decline in birth weights means men born in 2014 will on average grow to be just 170 centimeters tall and women only 157.9 centimeters, Morisaki’s team projected in a previous study.

Some think the culture is changing. The media are paying more attention to the problem of low birth weights, Fukuoka says, and dieticians and public health groups "are sounding alarms over undernourished young women." "There seems to be a trend in the fashion magazines, going from thinness to sportiness," Morisaki adds. The latest government survey shows the percentage of underweight women in their 20s has dropped slightly since 2013.

On the other hand, many slender pregnant women still post selfies on Instagram and share tips on managing weight gain. And most Japanese obstetricians are opposed to relaxing the weight gain recommendations, says Shunji Suzuki, an obstetrician at the Japanese Red Cross Katsushika Maternity Hospital in Tokyo. Japan's fascination with being thin hasn't quite run its course.

*Correction, 2 August, 12:15 p.m.: This story has been updated to correct the proportion of women entering pregnancy underweight and to correct a reference to a previous paper.