The weights of newborn babies have been declining for more than three decades as young pregnant women have been getting slimmer, according to the health ministry.
The average weight of baby boys at birth stood at 2,980 grams in 2010, down 61 grams from a decade earlier, while the figure for girls was 2,910, down 45 grams, a recent study by a health ministry research team showed. The two figures for 2010 saw a drop of 250 grams from their peaks in 1980.
Many studies say low weights at birth heighten the risks of babies developing diseases in adulthood. The ministry plans to study the issue.
Noriko Kato, research managing director at the National Institute of Public Health said the trend of newborns getting lighter for such a long period is rare among developed countries.
The health ministry research team found that lighter babies were born from thinner mothers. An increasing number of women are trying to curb their weight increase while pregnant.
That is apparently because of a 30-year-old education initiative that called on women to have smaller babies, as well as young women’s desire to be slim.
Having smaller babies was advocated when the weights of newborns were rising. The idea was to reduce health risks to pregnant women who gained too much weight and to promote easier deliveries.
A national survey in 2010 showed that 30 percent of women in their 20s were categorized as too thin, double the figure three decades earlier.
The ministry team cites other reasons for the smaller babies: Progress in medical science has lowered the death rates of underweight babies; the rate of women who smoke while pregnant has increased; the rates of women giving birth for the first time and multiple fetuses have also risen; and the pregnancy periods have been getting slightly shorter.
For reasons not understood, a mother's firstborn tends to weigh less than her subsequent babies.
“Another big factor for lighter newborns is that women are tending to become pregnant at older ages,” said Nobuya Unno, professor at Kitasato University and board member of the Japan Society of Obstetrics and Gynecology. “The risk of diabetes could rise by feeding small babies too much food. It’s not good for women to gain too much weight during pregnancy, but they should not be overly nervous about weight increases.”
In recent years, many studies, mainly from Western countries, have stated that lower weights at birth raise the risks of babies developing diabetes, high blood pressure and other adult diseases in the future.
Researchers suspect the environment when babies are in the womb could have an impact on their constitution.
A British survey found that there is roughly double the risk of men who were born with a weight under 2,500 grams dying of a heart attack, compared with men with a birth weight between 3,850 and 4,300 grams.
The health ministry will form a new research team in May that is expected to look into possible links between adults with and without adult diseases and their at-birth weights. It also plans to track the health of newborns until they become adults.
By YURI OIWA/ Staff Writer
The Asahi Shimbun