Americans Weigh in Over Time - Trends In U.s. Birth Weights
low birthweight obesity risk
Americans are not born overweight. In fact, the mean birth weight of infants born as singletons (births of one infant as opposed to twins or other multiple births) has steadily declined since 1990, according to the CDC National Center for Health Statistics. In 2003 the mean birth weight of all singletons was approximately 7 pounds, 5 ounces (3,325 g), and the average white non-Hispanic singleton (3,384 g; 7 pounds, 7 ounces) weighed a full nine ounces more than the average non-Hispanic black singleton (3,122 g; 6 pounds, 14 ounces). (See Table 1.2.) The percent of infants born with higher-than-average birth weights (4,000 g or more, or at least 8 pounds, 13 ounces) has been declining for more than a decade, as reported by the U.S. Department of Health and Human Services in Pediatric Nutrition Surveillance: 2003 Report. In 1994, 8.5% of births were at 4000 g or above, compared with 7.3% in 2003.
While ideal birth weight varies based on the expectant mother's ethnicity, for women in the United States, the average ideal birth weight is approximately 7.5 pounds, close to the average weight of singletons born in 2003. In the United States, the percent of babies born with low birth weight (LBW)—less than 2,500 g (5 pounds, 8 ounces) has risen steadily since the mid-1980s. (See Figure 1.2.) According to data from the CDC's National Center for Health Statistics the LBW rate rose from 7.6% in 2000 to 7.9% in 2003, the highest level reported in more than three decades. The percent of infants with very low birth weights (VLBW; weighing less than 1,500 g or 3 pounds, 5 ounces) remained nearly steady between 2000 (1.43%) and 2003 (1.45%).
LBW and VLBW are major predictors of infant morbidity (illness or disease) and mortality. For LBW infants, the risk of dying during the first year of life is more than five times that of infants born at normal weights; the risk for VLBW infants is nearly 100 times higher. The risk of delivering an LBW infant is greatest among the youngest and oldest mothers; however, many of the LBW births among older mothers are attributable to their higher rates of multiple births. CDC data for 2002 showed that multiples accounted for nearly two-thirds of all LBW infants delivered to mothers age forty-five and older that year. Close to 10% of singletons born to mothers age forty-five or older were LBW as were 8.7% of births to mothers less than twenty years old in 2002.
In 2003, 324,064 babies were born at low birth weights in the United States, according to the CDC (Births: Final Data for 2003). That number represented 7.9% of all births. However, the percent of LBW babies varied by state. In 2003 Alaska reported the lowest percent (5.2%) and Wyoming the highest (8.9%) of LBW births to non-Hispanic white mothers. Of the states that reported more than 1,000 births to non-Hispanic black women, LBW ranged from a low of 9.1% in Alaska to a high of 16% in New Mexico.
|Obesity prevalence among U.S. adults, by selected characteristics, 2001|
|SOURCE: Adapted from "Obesity and Diabetes Prevalence among U.S. Adults, by Selected Characteristics, BRFSS 2001," in Behavioral Risk Factor Surveillance System (BRFSS) (1985–2003), Centers for Disease Control and Prevention, Division of Nutrition and Physical Activity, National Center for Chronic Disease Prevention and Health Promotion, 2003, http://www.v/nccdphp/dnpa/obesity/trend/obesity_diabetes_characteristics.htm (accessed January 8, 2006)|
|Less than high school||27.4|
Birth Weight Influences Risk of Disease
Although the relationship between birth weight and development of disease in adulthood is an emerging field of research, and scientists cannot yet fully explain how and why birth weight is a predictor of health and illness in later life, mounting evidence indicates that both low birth weight and higher-than-average birth weight are linked to future health problems. Research reveals that LBW infants are more likely than normal-weight infants to develop disease in later life. Male infants with LBW who gain weight rapidly before their first birthdays appear to be at the highest risk. Investigators hypothesize that LBW infants have fewer muscle cells at birth and that rapid weight gain during the first year of life may lead to disproportionate amounts of fat to muscle and above average body mass. People with LBW who later develop above average body mass are at increased risk for developing diseases such as Type 2 diabetes, hypertension (high blood pressure), cardiovascular disease, and stroke. A 1997 study published in the British journal The Lancet examined the medical records of 13,249 men and found the risk of dying from stroke or heart disease was highest for those who weighed 5.5 pounds at birth or less. Those who weighed more than 8.5 pounds at birth had the lowest rate of mortality from heart attack or stroke.
|Rate of very low birthweight, low birthweight, and mean birthweight among singletons by race and Hispanic origin of mother, selected years 1990–2003|
|aData for 1990 by race and Hispanic origin exclude data for New Hampshire and Oklahoma, which did not require reporting of Hispanic origin of mother.|
|bIncludes births to races not shown separately.|
|cComputed in grams.|
|dIncludes persons of Hispanic origin of any race.|
|Notes: Very low birthweight is less than 1,500 grams. Low birthweight is less than 2,500 grams. Race categories are consistent with the 1977 Office of Management and Budget Guidelines.|
|SOURCE: Joyce A. Martin, Brady E. Hamilton, Paul D. Sutton, Stephanie J. Ventura, Fay Menacker, and Martha L. Munson, "Table H. Rate of Very Low Birthweight and Low Birthweight, and Mean Birthweight among Singletons by Race and Hispanic Origin of Mother: United States, 1990, 1995, 2000, and 2003," National Vital Statistics Reports, Births: Final Data for 2003, vol. 54, no. 2, Centers for Disease Control and Prevention, National Center for Health Statistics, September 8, 2005, http://www.cdc.gov/nchs/data/nvsr/nvsr54/nvsr54_02.pdf (accessed January 24, 2006)|
|Total, all races, originsb|
|Percent very low birthweight||1.11||1.11||1.11||1.08||1.05|
|Percent low birthweight||6.20||6.12||6.00||6.05||5.90|
|Mean birthweight in gramsc||3,325||3,332||3,348||3,353||3,365|
|Percent very low birthweight||0.82||0.81||0.80||0.78||0.73|
|Percent low birthweight||5.11||5.02||4.88||4.87||4.56|
|Mean birthweight in gramsc||3,384||3,392||3,410||3,416||3,433|
|Percent very low birthweight||2.61||2.63||2.62||2.55||2.54|
|Percent low birthweight||11.58||11.44||11.28||11.66||11.92|
|Mean birthweight in gramsc||3,122||3,128||3,141||3,132||3,128|
|Percent very low birthweight||0.94||0.96||0.94||0.93||0.87|
|Percent low birthweight||5.55||5.44||5.36||5.36||5.23|
|Mean birthweight in gramsc||3,324||3,332||3,344||3,343||3,351|
A 2005 study published in the American Heart Association Journal Circulation found an inverse relationship between birth weight and cardiovascular disease (heart disease and stroke). In general, rates of both coronary heart disease and stroke decreased with increasing birth weight. The association was strong, did not depend on adjustment for size in later childhood; and was independent of social class and other maternal and pregnancy characteristics.
Low birth weight also was linked to childhood asthma in a U.S. study published in a 2001 issue of Archives of Pediatrics and Adolescent Medicine, which found that babies born at 5.5 pounds or less faced the greatest risk of respiratory complications such as asthma. Research also has demonstrated that both LBW and abnormally high birth weight are associated with risk of developing diabetes later in life.
Evidence also indicates that birth weight is related to risk of developing breast cancer. Valerie A. McCormack and her colleagues at the Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, investigated whether size at birth and rate of fetal growth influenced the risk of developing breast cancer in adulthood. The results of the study were published in "Fetal Growth and Subsequent Risk of Breast Cancer: Results from Long Term Follow up of Swedish Cohort" (British Medical Journal, vol. 326, no. 7,383, February 2003). By examining birth and medical records of 5,358 singleton females born from 1915 to 1929, the investigators determined that size at birth was associated with breast cancer in premenopausal (the stage of reproductive life immediately before the onset of menopause) women age fifty or younger—the larger and longer the baby, the greater the risk. Birth weight or size was not associated with rates of breast cancer in postmenopausal women. Among premenopausal women who weighed 4,000 or more grams at birth (8 pounds, 14 ounces) rates of breast cancer were 3.5 times higher than those women who weighed less than 3,000 g at birth (about 6 pounds, 10 ounces). The investigators concluded that size at birth, including birth weight, length, and head circumference, is associated with risk of breast cancer in women under age fifty.
An analysis performed by Canadian researchers found that infants born either prematurely or with an extremely low birth weight (ELBW; 800 g or 1 pound, 12 ounces) were significantly more likely to suffer a lower level of fitness later in life, including less strength, endurance, and flexibility, and a greater risk of health problems as adults. When compared with teens born at normal weights, the ELBW teens had lower aerobic capacity, grip strength, leg power, and vertical jump. They were unable to perform as many push-ups, had less abdominal strength as measured by curl-ups, showed less flexibility in their lower backs, and had tighter hamstrings. The ELBW teens reported less previous and current sports participation, lower physical activity level, and poorer coordination compared with term-born control subjects. ELBW teens also had more trouble maintaining rhythm and tempo than their peers who were born at normal weights (Marilyn Rogers et al., "Aerobic Capacity, Strength, Flexibility, and Activity Level in Unimpaired Extremely Low Birth Weight [≤800 g] Survivors at Seventeen Years of Age Compared with Term-Born Control Subjects" Pediatrics, vol. 116, no. 1, July 2005).
The only action able to alter the birth weight of an infant is to modify weight gain during pregnancy. In 2006 health professionals concur that for normal-weight women the ideal weight gain during pregnancy ranges from twenty-five to thirty-five pounds of fat and lean mass. Further, research published in 2003 revealed that a newborn's birth weight and mother's post-pregnancy weight are influenced not only by how much weight is gained during pregnancy, but also by the source of the excess weight. In "Composition of Gestational Weight Gain Impacts Maternal Fat Retention and Infant Birth Weight" (American Journal of Obstetrics and Gynecology, vol. 189, no. 5, November 2003), researcher Nancy F. Butte and her colleagues conducted body scans of sixty-three women before, during, and after their pregnancies and recorded changes in women's weight from water, protein, fat, and potassium—a marker for changes in muscle tissue, one component of lean mass. The researchers found that only increases in lean mass, and not fat mass, appeared to influence infant size. Independent of how much fat the women gained during pregnancy, only lean body mass increased the birth weight of the infant, with women who gained more lean body mass giving birth to larger infants.
Breastfeeding is linked to improved health outcomes for all infants; however, it is especially advisable for LBW infants. For these infants, breastfeeding can reduce the risk that they will develop chronic diseases in adulthood by preventing the development of above average body mass. LBW infants who are breastfed for at least twelve months have about half the risk of developing above-average body mass during childhood.
FIRST WEEK OF LIFE MAY DETERMINE ADULT OBESITY
Research has demonstrated that low birth weight and low weight gain during infancy are associated with coronary heart disease. Similarly, rapid weight gain in infancy has been shown to predict obesity in childhood. In 2004 research funded by the National Institutes of Health (NIH) and conducted at the Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, and the Fomon Infant Nutrition Unit at the University of Iowa sought to determine which periods of weight gain in infancy might be associated with adult obesity.
The investigators reviewed data for 653 subjects who had been weighed on seven occasions during infancy and were contacted when they were young adults, ages twenty to thirty-two, when they again reported their heights and weight. The researchers pinpointed the period between birth and age eight days as potentially critical since weight gain during the first week of life was associated with adulthood overweight status. The formula-fed babies who gained weight rapidly during their first week of life were significantly more likely to be overweight decades later. They concluded that, "In formula-fed infants, weight gain during the first week of life may be a critical determinant for the development of obesity several decades later." The investigators also observed that their findings reinforced the American Academy of Pediatrics recommendation that infants should exclusively be breast-fed for the first six months of life. Among the numerous health benefits associated with breastfeeding is the fact that breast-fed babies are much less likely than formula-fed babies to become obese adults (Nicolas Stettler et al., "Weight Gain in the First Week of Life and Overweight in Adulthood: A Cohort Study of European American Subjects Fed Infant Formula," Circulation, vol. 111, no. 15, April 2005).
Another study, conducted by Janis Baird and her colleagues at the MRC Epidemiology Resource Centre at the University of Southampton in England found that big babies who grow quickly in the first two years of life risk being obese in childhood and adulthood. To determine whether obesity may begin in infancy, Baird and her colleagues looked at twenty-four studies that found an association between infant size or growth during the first two years of life and obesity later in life. They found that the heaviest infants and those who gained weight rapidly during the first and second year of life faced a nine-fold greater risk of obesity in childhood, adolescence, and adulthood. Their findings suggest that factors in infant growth are probably influencing the risk of later obesity. Baird and her colleagues do not know why big and fast growing babies had a higher risk of obesity, but they believe that some factors related to how an infant grows are important in influencing their later risk of obesity and suggested that infant feeding, being bottle or breast fed, the timing of weaning and social circumstances were factors that merit further investigation (Janis Baird et al., "Being Big or Growing Fast: Systematic Review of Size and Growth in Infancy and Later Obesity," British Medical Journal, vol. 331, no. 7,522, October 22, 2005).