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Pediatricians are alarmed at new statistics that show newborn infants are smaller on average than babies born 20 years ago.

According to the Health, Labor and Welfare Ministry, the average weight of newborns was 3,230 grams in 1980, but 3,060 grams in 2003.

The percentage of babies weighing less than 2,500 grams was 9.11 percent of all children born in 2003, up from 5.18 percent in 1980.

“Although women’s constitutions have improved, children are becoming smaller. This is an alarming situation,” said Kazuo Itabashi, a professor specialized in newborn care at Showa University. “Unless a baby grows to a sufficient size in the mother’s womb, an infant will develop various diseases after growing up.”

Infants born 10 percent or more below standard weight and length are “small for gestational age” babies, or SGA.

Some of the causes are rubella and other infections, congenital abnormalities, malnourishment in mothers from excessive dieting, and excessive drinking and smoking by pregnant women.

“No clear-cut reason for the underweight trend is known, but some of the reasons being considered are a woman’s dieting and childbearing in later years,” he said. “What is important is appropriate nutritional guidance for pregnant women and breast-feeding for as long as possible.”

About 90 percent of underweight infants grow rapidly until the age of 3, but will become short adults unless treated with growth hormones.

However, “After growth, SGA infants are liable to develop metabolic syndrome,” Itabashi said. Metabolic syndrome is also known as insulin resistance syndrome.

SGA infants also have less muscle and bone than normal infants.

In addition, leptin, a neurotransmitter that curbs appetite, does not function properly, so SGA children are liable to eat more, which increases their chances of suffering from obesity, diabetes, high blood pressure and arterial sclerosis.

Itabashi cited an example of an 8-year-old child with a fatty liver — a problem more likely to be found in a heavy drinker or obese adult and which can lead to cirrhosis.

A U.S. epidemiological survey found that underweight newborns who grew rapidly until the age of 11 had a greater chance of developing diabetes and coronary problems as adults.

Itabashi urged parents to monitor their children’s growth for abnormalities and to take precautions with their diet.