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America's Children and the Environment (ACE)

ACE: Health - Adverse Birth Outcomes

Indicator H12

Updated August 2019Percentage of babies born preterm, by race/ethnicity, 1993-2017Data Tables for this Topic

Data characterization

  • Data from this indicator are obtained from a database maintained by the National Center for Health Statistics.
  • The database collects information from birth certificates for virtually all births in the United States.
  • Length of gestation is recorded on each birth certificate.
  • Between 1993 and 2006, the rate of preterm birth increased from 11.0% in 1993 to its highest value of 12.8% in 2006. This increasing trend was statistically significant. The rate of preterm birth decreased from 10.4% in 2007 to 9.6% in 2014, and was 9.9% in 2017. The overall decreasing trend from 2007 to 2017 was statistically significant.
    • Values from 2007 to the present are not comparable to those for earlier years due to a change in the measure for estimating gestational age.
  • In 2017, Black non-Hispanic women had the highest rate of preterm birth, compared with women of other races/ethnicities. About 1 in 7 infants born to Black non-Hispanic women were born prematurely in that year.
    • The differences between the rate of preterm birth for Black non-Hispanic women and the rates for the other race/ethnicity groups were statistically significant.
  • Between 2007 and 2017, the preterm birth rate showed a decreasing trend for each race/ethnicity group except American Indian Alaska Native non-Hispanic women. The preterm birth rate for American Indian Alaska Native non-Hispanic women showed an increasing trend.
    • These trends in the rate of preterm birth were statistically significant for each race/ethnicity group.
  • The preterm birth rate varies depending on the age of the mother. Women ages 20 to 39 years have the lowest rate of preterm birth, compared with women under 20 years and women 40 years and older.
    • The differences between the preterm birth rates for the different age groups were statistically significant. The decreasing trends from 2007 to 2017 in the rate of preterm birth for women were statistically significant for each age group.
  • Twins, triplets, and other higher-order multiple birth babies are more than 5 times as likely to be born preterm compared with singleton babies (e.g., 53.1% vs. 9.9% in 1993 and 60.6% vs. 8.1% in 2017). The preterm birth rates for both singletons and multiples showed an increasing trend from 1993 to 2006 followed by a decreasing trend from 2007 to 2017; however, the trends for multiples were larger than for singletons. (See Table H12b.)
    • These trends for both singleton and multiple births were statistically significant.

Indicator H13

Updated August 2019Percentage of babies born at term with low birth weight, by race/ethnicity, 1993-2017Data Tables for this Topic

Data characterization

  • Data from this indicator are obtained from a database maintained by the National Center for Health Statistics.
  • The database collects information from birth certificates for virtually all births in the United States.
  • Birth weight and length of gestation are recorded on each birth certificate.
  • Between 1993 and 2006, the rate of term low birth weight for all races/ethnicities stayed relatively constant, ranging between 2.5% and 2.7%.  Between 2007 and 2017, the rate of term low birth weight for women of all race/ethnicities increased from 2.4% to 2.6%.  This increasing trend was statistically significant.
    • Values from 2007 to the present are not comparable to those for earlier years due to a change in the measure for estimating gestational age.
  • Between 2007 and 2017, the rates of term low birth weight increased for each race/ethnicity group except for women of unknown ethnicity. Each of these increasing trends was statistically significant.
  • The rate of term low birth weight varies by race/ethnicity. In 2017, the rate was highest for Black non-Hispanic women (4.3%), and next highest for Asian or Pacific Islander non-Hispanic women (3.2%). The rate of term low birth weight is lowest for White non-Hispanic women (2.1%), Mexican women (2.0%), and Hispanic women (2.2%).
    • The rate of term low birth weight for Black non-Hispanic women was statistically significantly higher than for all other race/ethnicity groups. The rate of term low birth weight for Asian or Pacific Islander non-Hispanic women was significantly lower than for Black non-Hispanic women but significantly higher than the other race/ethnicity groups.
  • Term low birth weight rates vary by the age of the mother. In 2017, women ages 20 to 39 years had the lowest rate of term low birth weight infants, while women under 20 years had the highest rate of term low birth weight infants. These differences were statistically significant. (See Table H13a.)
  • Between 2007 and 2017, the rates of term low birth weight for women under 20 and women ages 20 to 39 years showed increasing trends. These trends were statistically significant. (See Table H13a.)
  • Twins, triplets, and other higher-order multiple birth babies are more than 4 times as likely to be born at term with low birth weight compared with singleton babies (e.g., 13.4% vs 2.3% in 1993 and 9.6% vs. 2.3% in 2017). The rate of term low birth weight for singleton babies stayed relatively constant over the period 2007-2017. The rate of term low birth weight for multiple birth babies also stayed constant over the period 2007-2017. (See Table H13b.)

About the Adverse Birth Outcomes Indicators

Indicators H12 and H13 present information about babies born preterm and those born with low birth weight. The data are from a vital statistics registration system that registers vital events including virtually all births in the United States.

The period of gestation is a crucial determinant of an infant's health and survival for years to come. Two measures that may be used to understand the quality of an infant's gestation are length of gestation (pregnancy length) and birth weight. Normal term pregnancies last between 37 and 41 completed weeks: preterm birth is defined as a live birth before 37 completed weeks of gestation. Birth weight is determined by length of gestation and fetal growth (the rate at which an infant develops and increases in size). Low birth weight infants are defined as weighing less than 2,500 grams (about 5 pounds, 8 ounces).

A growing number of studies have examined the possible role that exposure to environmental contaminants may play in the causation of preterm birth and low birth weight. The Surgeon General has determined that exposure of pregnant women to environmental tobacco smoke (ETS) causes a small reduction in mean birth weight and the evidence is suggestive (but not sufficient to infer causation) of a relationship between maternal exposure to environmental tobacco smoke during pregnancy and preterm delivery. The Surgeon General has stated that there is no risk-free level of exposure to ETS. Additionally, the National Toxicology Program has concluded that maternal exposure to lead is known to cause reduced fetal growth, and that there is limited evidence of an association with preterm birth. Finally, some studies have reported associations between air pollution or other environmental contaminants and adverse birth outcomes. Preterm and low birth weight infants are at greater risk for mortality and a variety of health and developmental problems, both in childhood and adulthood.

Indicators H12 and H13 present data on preterm birth and low birth weight from the National Vital Statistics System (NVSS).

More information about obesity and Indicators H12 and H13 is provided in the Adverse Birth Outcomes section of America's Children and the Environment, Third Edition.

Related Links

Centers for Disease Control and Prevention (CDC): Maternal and Child Health Exit

Institute of Medicine (IOM): Preterm Birth: Causes, Consequences, and Prevention

National Toxicology Program (NTP): Health Effects of Low-Level Lead Evaluation

U.S. Department of Health & Human Services (HHS): The Health Consequences of Involuntary Exposure to Tobacco Smoke: A Report of the Surgeon General

U.S. EPA Integrated Science Assessment: Particulate Matter (PM)

Summary of Methods - Adverse Birth Outcomes

The National Center for Health Statistics, a division of the Centers for Disease Control and Prevention (CDC), maintains the National Vital Statistics System (NVSS) which compiles national data on gestational ages and birth weights from birth certificate registration data from states and other U.S. jurisdictions. The NVSS data are provided through contracts between the CDC and vital registration systems operated in each state, which are legally responsible for the registration of vital events including births, deaths, marriages, divorces, and fetal deaths. Birth certificates provide information on characteristics of both the infant and his/her parents, including the weight of the infant and the length of gestation. Length of gestation is recorded in completed weeks and is based on clinical estimation, usually from the date of the woman's last menstrual period or ultrasound.

Indicator H12 uses the NVSS data to present the percentage of babies born preterm (defined as a period of gestation less than 37 completed weeks) stratified by race/ethnicity. Indicator H13 uses the data to present the percentage of babies that are both born at term (defined as a period of gestation of at least 37 completed weeks) and have low birth weight (defined as a weight less than 2,500 grams), stratified by race/ethnicity.

Detailed Methods for Indicators H12 and H13

Metadata for National Health Interview Survey (NHIS)