Women who gain more than 40 pounds (18 kg ) during pregnancy have nearly twice the risk of delivering a heavy baby as those who gain less, US researchers said on Friday.

The study of more than 40,000 U.S. women and their babies found as many as one in five women gains too much weight during pregnancy, doubling the chances her baby will weigh 9 pounds (4 kg) or more.

And they found women who gain more than 40 pounds during pregnancy are more likely to have a heavy baby even if they do not have gestational diabetes, a short-term form of diabetes linked with pregnancy that is known to increase the risk of having a big baby.

"Because there are so many women who are gaining more than 40 pounds during pregnancy, it's an important health message for most women to avoid excessive weight during pregnancy," Dr. Teresa Hillier of Kaiser Permanente Center for Health Research in Portland, Oregon, whose study appears in the journal Obstetrics & Gynecology, said in a telephone interview.

Hillier said gaining extra weight during pregnancy increases the risk for having heavy babies, and studies suggest these babies are programed to become overweight or obese later in life.

According to the American College of Obstetricians and Gynecologists, babies who weigh more than 9 pounds at birth are considered heavy.

A large baby can pose risks for a difficult delivery -- increasing the chances of vaginal tearing, bleeding, and Caesarian-sections for the mother and the risk of stuck shoulders and broken collar bones for the baby.

In the study, Hillier and colleagues examined the medical records of 41,540 women who gave birth in Washington, Oregon and Hawaii from 1995-2003. All had been tested for gestational diabetes and 5.4 percent were treated for it with a program of diet, exercise and insulin, if needed, to control blood sugar.

Overall, 20 percent of the women in the study who gained more than 40 pounds -- the upper limit of pregnancy weight gain recommendations in the United States -- gave birth to heavy babies.

Fewer than 12 percent of women in the study with normal weight gain delivered heavy babies, she said.

The group at greatest risk were those who gained more than 40 pounds and had gestational diabetes. Nearly 30 percent of these women had heavy babies, compared with 13.5 percent of women with gestational diabetes who had normal weight gains during pregnancy.

The researchers said the findings suggest all women should avoid excessive weight gain during pregnancy. And women who are being treated for gestational diabetes should also strive to keep weight gain below 40 pounds.

"You can't treat the glucose and ignore the weight gain issue," Hillier said.

Weight gain during pregnancy has been rising over the past two decades, and some researchers suspect this may be fueling an epidemic of childhood obesity.

美國研究人員于上周五稱，懷孕期間體重增幅超過40磅（18公斤）的女性生出超重嬰兒的幾率幾乎是增重較少女性的兩倍。

該研究共對美國4萬多個女性和她們的寶寶進行了調查。調查結果顯示，其中多達五分之一的女性懷孕期間體重增長過多，這使她們的孩子出生體重達或超過9磅（4公斤）的幾率增加了一倍。

此外，研究人員發現，孕期體重增加超過40磅的女性即使沒有患妊娠糖尿病，也更易生出體重超標的嬰兒。妊娠糖尿病是孕婦在妊娠期間易患的一種臨時性的糖尿病，該病會增加孕婦生出超重嬰兒的風險。

位于俄勒岡州波特蘭市的凱薩健康研究中心的特里薩• 希利爾博士在電話采訪中說：“現在有很多孕婦孕期體重增幅超過40磅，所以對于大多數女性來說，避免孕期增重過度是一個很重要的健康信息。”該研究結果在《產科和婦科醫學》上發表。

希利爾稱，孕期增重過多會增加生出超重嬰兒的風險。此外，研究表明，這些孩子日后體重超標或肥胖的可能性也較大。

根據美國婦產科醫師協會的標準，嬰兒出生時體重超過9磅即為超重。

胎兒過大會增加難產的風險——產道撕裂、大出血、剖腹產以及嬰兒肩難產和鎖骨骨折的幾率增大。

在研究過程中，希利爾及其同事對華盛頓、俄勒岡和夏威夷1995年至2003年間的41540名孕婦的病歷進行了研究。這些孕婦都接受了妊娠糖尿病檢測，其中5.4%的人通過飲食、運動或注射胰島素（如果需要的話）控制血糖，治療糖尿病。

總體來看，在孕期體重增幅超過40磅的女性中，有20%的人生下了超重嬰兒。在美國，孕期增重建議最高上限為40磅。

她說，孕期體重增幅在正常范圍內的女性中，生下了超重嬰兒的不到12%。

其中最危險的群體是孕期增重超過40磅且患有妊娠糖尿病的人，她們當中近30%的人生出了超重嬰兒，而這一數字在體重增幅正常、患妊娠糖尿病的孕婦中僅為13.5%。

研究者稱，該結果表明，所有女性都要避免孕期增重過度，而且正在接受妊娠糖尿病治療的孕婦也要盡量將體重增幅控制在40磅以內。

希利爾說：“孕婦不能只顧控制血糖，而忽視體重增加的危險。”

在過去20年中，女性孕期增重過度的問題越來越嚴重。一些研究者認為，這可能會導致兒童肥胖的流行。