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.

美国研究人员于上周五称，怀孕期间体重增幅超过40磅（18公斤）的女性生出超重婴儿的几率几乎是增重较少女性的两倍。

该研究共对美国4万多个女性和她们的宝宝进行了调查。调查结果显示，其中多达五分之一的女性怀孕期间体重增长过多，这使她们的孩子出生体重达或超过9磅（4公斤）的几率增加了一倍。

此外，研究人员发现，孕期体重增加超过40磅的女性即使没有患妊娠糖尿病，也更易生出体重超标的婴儿。妊娠糖尿病是孕妇在妊娠期间易患的一种临时性的糖尿病，该病会增加孕妇生出超重婴儿的风险。

位于俄勒冈州波特兰市的凯萨健康研究中心的特里萨• 希利尔博士在电话采访中说：“现在有很多孕妇孕期体重增幅超过40磅，所以对于大多数女性来说，避免孕期增重过度是一个很重要的健康信息。”该研究结果在《产科和妇科医学》上发表。

希利尔称，孕期增重过多会增加生出超重婴儿的风险。此外，研究表明，这些孩子日后体重超标或肥胖的可能性也较大。

根据美国妇产科医师协会的标准，婴儿出生时体重超过9磅即为超重。

胎儿过大会增加难产的风险——产道撕裂、大出血、剖腹产以及婴儿肩难产和锁骨骨折的几率增大。

在研究过程中，希利尔及其同事对华盛顿、俄勒冈和夏威夷1995年至2003年间的41540名孕妇的病历进行了研究。这些孕妇都接受了妊娠糖尿病检测，其中5.4%的人通过饮食、运动或注射胰岛素（如果需要的话）控制血糖，治疗糖尿病。

总体来看，在孕期体重增幅超过40磅的女性中，有20%的人生下了超重婴儿。在美国，孕期增重建议最高上限为40磅。

她说，孕期体重增幅在正常范围内的女性中，生下了超重婴儿的不到12%。

其中最危险的群体是孕期增重超过40磅且患有妊娠糖尿病的人，她们当中近30%的人生出了超重婴儿，而这一数字在体重增幅正常、患妊娠糖尿病的孕妇中仅为13.5%。

研究者称，该结果表明，所有女性都要避免孕期增重过度，而且正在接受妊娠糖尿病治疗的孕妇也要尽量将体重增幅控制在40磅以内。

希利尔说：“孕妇不能只顾控制血糖，而忽视体重增加的危险。”

在过去20年中，女性孕期增重过度的问题越来越严重。一些研究者认为，这可能会导致儿童肥胖的流行。