(CNN) - Naomi Hayami-Chisuwa, who gave birth in June at age 35, found it hard to control her weight during pregnancy, particularly as she came closer to the due date.
Despite being a normal weight, the dietitian and lecturer at Osaka City University had been advised by the hospital where she received health care to gain only 10 kilograms until delivery, slightly less than the official government guideline of 7 to 12 kilograms (about 15 to 26 pounds), she said.
Tipping the scales proved stressful to this working mother-to-be, who could not attend daytime pregnancy education classes and so was advised to "take nutrition counseling twice when my weight increased over the recommended weight gain per week."
For the sessions, she created a record of her diet and this was examined by nurses "to find eating behaviors which may be related to weight gain," she said. In her case, no specific recommendations were given because her diet and behaviors were healthy.
Other pregnant women probably face the same even though these sessions are not mandated by law, she said. Pregnant women register their pregnancies at a local government office where they receive a Maternal and Child Health handbook, which was devised 70 years ago. The handbook, which treats maternal and child health as one, is a record of prenatal checkups, delivery, child development and vaccinations, and it serves the same purpose, more or less, as a portable electronic health record. Using the handbook, obstetricians closely track a woman's prenatal weight gain.
"In my view, doctors, nurses and midwives (especially doctors) are strict about weight gain," Hayami-Chisuwa wrote in an email. "The reason is mainly to prevent pregnancy-induced hypertension syndrome." High blood pressure during pregnancy can lead to complications during birth.
Yet maintaining a low weight during pregnancy may damage a baby's future health and affect the overall health of the nation, one researcher believes.
Insufficient weight can cause health problems
"Numerous studies have shown that lower gestational weight gain causes lower birthweight," Dr. Naho Morisaki, chief of the Division of Lifecourse Epidemiology in the Department of Social Medicine at Japan's National Center for Child Health and Development, wrote in an email.
Pregnant women who don't gain enough weight may deliver babies who are too small, leading to difficulties breastfeeding, increased risk of illnesses and trouble meeting developmental milestones, according to the US Centers for Disease Control and Prevention.
What's the recommended weight gain?
Japan's Ministry of Health, Labour and Welfare recommends a gain of 9 to 12 kilograms (about 20 to 26 pounds) for underweight women (body mass index 18.5 or less) and 7 to 12 kilograms (about 15 to 26 pounds) for normal-weight women (BMI 18.6 to less than 25). Overweight women (BMI 25 to 30) need to add 7 kilograms or less (about 15 pounds), while obese women (BMI greater than 30) should gain only 5 kilograms or less (about 11 pounds).
US guidelines from the Institute of Medicine recommend that underweight women gain 28 to 40 pounds during pregnancy (12.5 to 18 kilograms); normal-weight women gain 25 to 35 pounds (11.5 to 16 kilograms); overweight women gain 15 to 25 pounds (7 to 11.5 kilograms); and obese women gain 11 to 20 pounds (5 to 9 kilograms).
The differing guidelines have a tangible impact. Japanese women give birth to infants 300 grams (about 0.7 pounds) smaller, on average, than white mothers, Morisaki said. This is a direct result of low gestational weight gains.
Data from 2010 indicates that nearly 20% of Japanese women are gaining less weight than the guidelines suggest, Morisaki noted.
"Japanese have extremely low gestational weight gain among thin and normal BMI women, which explains why birthweight is low even compared to other east Asians who also generally tend to be thin like Japanese," he said.
Hayami-Chisuwa noted that the official ministry guideline may be disregarded by some doctors who recommend even stricter weight gain guidelines -- as happened in her case.
Applying US standards to Japan, 68% of women are not gaining enough, Morisaki said: "If the Japanese guidelines are not right, we may have a big problem."
The ministry has formed a task team to investigate whether the gestational weight gain guidelines, released in 2001, are scientifically sound.
A member of the team, Dr. Kyoko Nomura, a professor in the Department of Public Health at Akita University's Graduate School of Medicine, wrote in an email that her team's research compared the impact of US Institute of Medicine and Japanese guidelines.
The 2017 study examined rates of cesarean sections and low and high birth-weight babies among women who stayed within the Japanese guidelines and compared those with women who stayed within US guidelines.
The results suggest that Japanese guidelines are "fair enough and do not need to change" since differences were negligible, she said.
But this does not mean both guidelines are equally beneficial for everyone, she noted: "The IOM guidelines are useful for Western countries, where overweight and obese women are more prevalent; however, Asian women are smaller, and their gestational weight gain is different from that of women of other ethnicities."
"Japan currently faces a serious problem of an increasing number of low birth weight babies," Nomura said. Before 1990, Japan's prevalence of low birth-weight babies was below the 6.5% average found in member countries of the Organization for Economic Cooperation and Development, she explained. However, over time, the number of low birth-weight babies increased, and Japan hit a 9.5% prevalence rate in the 2018 survey.
"One of the biggest reasons" for this increasing rate is the fact that one-fourth of Japanese women of reproductive age are underweight, Nomura said. As a result, she emphasizes "the importance of staying in normal weight ranges for all women who are in reproductive ages."
Low birth weight rate might affect life expectancy
Studies indicate that lower baby birth weights increase the risk of chronic diseases later in life. Morisaki's own research shows how a mother's gestational weight gain is associated with both a baby's birth weight and its height in adulthood.
"Adult height, which is linked to higher risk of chronic diseases in large studies in and out of Japan, can be an indicator for lifelong health," he explained.
The rate of low birth-weight babies and adult height shows a strong inverse correlation, he said. Due to a trend in smaller babies, adult stature in Japan has started to decline among those born after 1980. Creating a prediction model, he estimated that the national average of adult height would decline about 0.7 centimeters (about a quarter of an inch) from the current average of 170.7 centimeters (about 5 feet 6 inches) for men born in 2014.
Based on these figures, he also speculated that there will be 3% more cardiovascular disease-related deaths (roughly 6,000 deaths) among Japanese men born in 2014 compared with those born in 1980.
"This may also affect Japanese life expectancy," Morisaki said. Japan, where citizens can expect to live an average of 84.2 years, prides itself on its No. 1 global ranking on this issue.
Fearing for a future in which life expectancy dwindles, Morisaki conducted a questionnaire study among pregnant women to better understand why some are reluctant to gain weight.
"Japanese women tend to self-restrict weight gain even lower than current recommendations, as they think that doing so will 'ease delivery' and 'help them retain a better body proportion after delivery,' " he said.
Yet these self-imposed restrictions do nothing for the women, he said: Slimmer pregnancies did not decrease the risk of a c-section and did not change the number of extra pounds still retained a year after birth.
Hayami-Chisuwa believes that more flexibility is needed on the issue of pregnancy weight gain, since some women who are at lower than average weight may not take in "enough nutrition for an unborn baby."
"Health professionals need to pay attention to not only weight gain, but also [cases in which women] are underweight before pregnancy."
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