The majority of women gain too much or too little weight during pregnancy, putting both mothers and babies at an increased risk for complications, a review of past studies suggests.
Researchers examined data from 23 studies covering a total of more than 1.3 million pregnancies and found 47 percent of the time women gained more weight than recommended by the Institute of Medicine (IOM)—about 25 to 35 pounds for people who start out pregnancy at a normal weight. Another 23 percent of the time, women didn’t gain enough weight.
Too little pregnancy weight gain was associated with a higher risk of undersized and premature infants, while too much weight gain was linked to greater odds of oversized babies and cesarean section or surgical deliveries, researchers report in JAMA.
"Women are rarely aware of healthy weight gain targets in pregnancy and are not generally weighed or supported to reach them," said senior study author Dr. Helena Teede of Monash University in Victoria, Australia.
"This clearly needs to change," Teede said by e-mail. "Based on the study results both mothers and babies are healthier with less complications like cesarean sections and less small or large babies if weight gain is within targets.”
Weight gain recommendations vary depending on women’s size when they become pregnant. Women who are underweight should gain 28 to 40 pounds, according to the IOM. For overweight women, a 15- to 25-pound gain is recommended and obese women should gain just 11 to 20 pounds.
To see how often women were below, within or above recommended weight ranges for pregnancy, researchers assessed weight gains across all of the pregnancies based on women’s pre-pregnancy body mass index (BMI), a measure of weight relative to height.
An adult who is 5 feet 9 inches tall and weighs from 125 to 168 pounds would have a BMI of 18.5 to 24.9 and be considered a healthy weight, according to the U.S. Centers for Disease Control and Prevention. An obese adult at that height would have a BMI of 30 or more and weigh at least 203 pounds.
Compared with women whose weight gain fell within a range recommended by the IOM, women who added too few pounds during pregnancy were 70 percent more likely to have premature deliveries and 53 percent more likely to have babies that were small for their gestational age, the study found.
Too little weight gain didn’t appear to influence their risk of a cesarean section, and it was linked to lower odds of having overweight babies or infants whose shoulders got stuck in the birth canal during delivery.
But compared with women who gained a recommended amount of weight, women who added too many pounds were 85 percent more likely to have babies that were large for their gestational age and 30 percent more likely to have cesarean deliveries.
The study wasn’t a controlled experiment designed to prove how the amount of pregnancy weight gain influences specific health outcomes for mothers or babies.
Another limitation is that the individual studies had inconsistent definitions for certain outcomes like underweight or overweight infants or preterm deliveries, the authors note. Researchers could not always distinguish, for example, between a scheduled or elective cesarean delivery and an emergency surgery, and they couldn’t tell when preterm births occurred naturally or were induced.
The analysis also included studies published both before and after the IOM weight gain guidelines came out in 2009, and weight gain targets may have differed across the studies, the authors point out.
Even so, the findings confirm and strengthen results from many smaller studies suggesting that many women gain more than the recommended amount of weight during pregnancy, said Dr. Aaron Caughey, chair of obstetrics and gynecology at Oregon Health and Science University in Portland and author of an accompanying editorial.
"Even in individuals who might normally eat reasonable amounts, the hormones of pregnancy often lead to greater food consumption," Caughey said by e-mail. "Because of this ready availability of high caloric foods, it is easy to gain too much weight, which is the most common outcome."