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Low-weight births in Japan spark new guidelines on weight of moms to curb risks

A doctor explains the risks of being underweight to pregnant women during a maternity class held online at Kato Clinic in the city of Saitama. (Mainichi/Ayumu Iwasaki)

TOKYO -- Japan is seeing a trend in declining birth weights of infants, which can threaten their health, possibly due to pregnant women not gaining enough weight.

    The trend has prompted a committee of the Japan Society of Obstetrics and Gynecology (JSOG) to compile new guidelines on the appropriate body weights of expectant mothers as early as this spring.

    The Mainichi Shimbun looked into why low birth weight (LBW) is actually problematic, in spite of the saying in Japan that "mothers should give birth to small babies and raise them to be big."

    Japan has seen the average birth weight of infants decline since the late 1970s. The average weight was 3,195 grams in 1975 and 3,005 grams in 2019, according to the Ministry of Health, Labor and Welfare.

    The ratio of LBW infants, who weigh less than 2,500 grams, increased from 5.1% in 1975 to around 9.5% from 2005 onward. According to data compiled by the Organisation for Economic Co-operation and Development (OECD) in 2018, this figure is the second largest following Greece among all member countries.

    In response to this situation, which is unusual for an advanced nation, the JSOG decided to formulate new guidelines on how much weight a pregnant woman should gain before giving birth.

    It has been pointed out that one of the reasons for the increase of LBW babies is due to the rise in the number of underweight women in Japan. In 2018, 19.8% of women in their 20s and 19.3% in their 30s fell into the underweight category. This standard for being underweight is having a body mass index (BMI) -- defined as a person's weight in kilograms divided by the square of their height in meters -- of under 18.5. The figures suggest many women of childbearing age are underweight.

    The JSOG created guidelines on the appropriate body weight of pregnant women in 1997. However, this was not out of concern for the health of the fetuses, but with the aim of preventing pregnancy-induced high blood pressure. It said women with a BMI of under 18 before pregnancy should gain 10 to 12 kilograms; those with levels between 18 to 24 should gain 7 to 10 kilograms: and people with a BMI exceeding 24 should gain 5 to 7 kilograms.

    In 2006, amid a national movement aiming to improve the health of mothers and children, similar indicators were announced by the health ministry, aiming to keep the birth weight of newborns between 2,500 and 4,000 grams. This is still stated in the maternal and child health record book local governments distribute to pregnant women.

    However, it was pointed out that the JSOG and ministry's numerical standards on how much weight underweight women should gain were too low.

    According to analysis of data on at least 100,000 pregnant women made public in 2017 by researchers at the National Center for Child Health and Development, expectant mothers with a BMI of under 18.5 should gain 12.2 kilograms (permissible range from 10.8 to 13.6 kg) to prevent undergrowth or overgrowth of their fetuses. The figures indicate underweight women should gain more weight than what has been described in the past.

    Naho Morisaki, head of the center's life course epidemiology laboratory, pointed out, "The thinner pregnant women are, the more they tend to keep their weight down out of consideration for their body shape after giving birth." She added, "The existing indicators show a wide range of weight gain, and people who strongly wish to stay thin choose the lowest figure as their goal. This could further contribute to their lack of weight gain."

    In response to the increase in LBW infants and other related issues, the JSOG decided in 2019 to not recommend using the indices formulated in 1997. It will formulate new guidelines based on the analysis of data on 500,000 to 600,000 women who gave birth between 2015 and 2017. The new indices -- which will be released as early as March 2021 -- will be based on information including the body weight of women before and after giving birth, the number of weeks of pregnancy, and the birth weight of their babies.

    The aim of managing one's body weight when pregnant is not only to prevent pregnant women from becoming too skinny, but also to avoid emergency C-sections due to gaining too much weight, and to prevent premature delivery. Since many women in Japan give birth at a relatively small clinic where it is hard to conduct an emergency C-section, it is important to prevent expectant mothers from putting on too much weight.

    Atsuo Itakura, a JSOG committee representative and OB-GYN professor at Juntendo University, commented, "The birth weight of babies has declined by nearly 200 grams over the last 40 years, and we need countermeasures. We will decide on the indices while considering objectives besides decreasing the number of LBW newborns."

    It has been pointed out that one reason birth weight is so important -- to the extent that the JSOG has to formulate new guidelines on the body weights of pregnant women -- is because the health and nutritional status of a newborn can have effect on their health when they grow older.

    Epidemiological surveys showing LBW infants had a high chance of developing diabetes, high blood pressure, hyperlipidemia and other conditions once they become adults were repeatedly reported overseas in the 1980s and '90s.

    The bodies of malnourished fetuses apparently expect that they will keep on suffering from a lack of nutrition after they are born, and become likely to store energy by adjusting their blood circulation and metabolism. Therefore, when such people take in the appropriate amount of nutrition after birth, they are said to be actually taking in too much -- which is likely to increase the risk of lifestyle-related illnesses.

    Japan also has data on the correlation between birth weight and the risk of high blood pressure after such children grow up. A study released in 2006 by a research team of Nagoya University analyzed the relationship between weight at birth and blood pressure of 3,107 office workers aged between 35 and 66. The results showed that those who weighed less than 2,500 grams at birth carried a 1.26 times higher risk of developing high blood pressure than those who weighed between 2,500 and 3,000 g. Meanwhile, people who weighed between 3,000 and 3,500 grams at birth carried a 0.89 times lower risk.

    In response to such research, the American journal Science Magazine published an article in 2018 that warned Japan could face long-term health problems due to its many LBW infants.

    Though epidemiological survey results exist, it takes a long period to actually investigate the lifestyles of LBW babies and confirm their health status after they reach adulthood. Professor Itakura says that the JSOG's new indicators will aim to reduce future health risks for children by managing the body weight of expectant mothers and their children's birth weight, as the best method there is at the time.

    Morisaki of the child health and development center emphasized, "It (the issue of LBW babies) could increase Japan's health costs in the future. People who had a low birth weight should be aware of the risks and be cautious about their lifestyles."

    (Japanese original by Ryo Watanabe and Ayumu Iwasaki, Science & Environment News Department)

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