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Frequently Asked Questions: Birth

Why are we concerned about pollution and environmental exposures?

The fetus is developing along with critical organ systems during pregnancy. There are critical windows of development where environmental exposures could damage growth and function.

What is prematurity?

Preterm birth (birth at less than 37 completed weeks of gestation) affects more than 500,000 or 12.5% of live births in the US and is a leading cause of infant morbidity and mortality (sickness and death, respectively). The majority of premature babies (about 84%) are born moderately preterm (between 32 and 36 completed weeks of gestation). About 16% are born very preterm (at less than 32 weeks of gestation); of those, about 10% are born between 28-31 weeks of gestation and about 6% are born at less than 28 weeks of gestation.
Preterm birth is a leading cause of infant mortality, morbidity, and long-term disability. All infants born preterm are at risk for serious health problems; however, those born earliest are at greater risk of medical complications, long-term disabilities, and death.  

Why should we be concerned about preventing premature births?

Preterm infants are at greater risk of serious health problems for several reasons. The earlier is a baby is born, the less it will weight, the less developed its organs will be, and the more medical complications it will likely face later in life. Very preterm infants have the greatest risk of death and lasting disabilities, including mental retardation, cerebral palsy, respiratory and gastrointestinal problems, and vision and hearing loss. Preterm births account for health care expenditure of over $3 billion per year.

What causes preterm birth?

Studies have shown that major risk factors associated with preterm birth include:
  • Plural births (twins, triplets…)
  • Previous preterm birth
  • Certain uterine or cervical abnormalities of the mother
  • Mother’s age, race, poverty (for example, African-American women, women younger than 17 and older than 35, and poor women are at greater risk than other women)
  • Male (associated with singleton preterm birth)
  • Certain lifestyles and environmental factors, including:
-Maternal lifestyle factors, including:
·         Obesity
·         Late or no prenatal care
·         Smoking
·         Alcohol consumption (especially, in early pregnancy)
·         Illegal drug use
·         Exposure to the medication diethylstilbestrol (DES)
·         Domestic violence
·         Lack of social support
·         Stress (including long working hours, or long periods of standing)
·         Underweight before pregnancy
·         Birth spacing less than 6-9 months between birth and the beginning of the next pregnancy
-Neighborhood-level characteristics (e.g., high poverty levels and high levels of deprivation)
-Environmental contaminants (e.g., exposure to air pollution and drinking water contaminated with chemical disinfection by-products or lead).
Certain medical conditions during pregnancy (e.g., infections, diabetes, hypertension, blood clotting disorders/thrombophilia, bleeding from the vagina, certain birth defects of the fetus, being pregnant with a single fetus following in vitro fertilization) may also increase the risk of preterm birth. 

What is low birthweight?

An infant is considered to be of low birthweight when its weight is less than 2500 grams (5 lbs. 8 oz) at birth. A low birthweight infant can be born too soon (premature) or too small (growth retarded) or both. Thus, low birthweight is a category which contains both premature and growth retarded infants.

Why should we be concerned about preventing low birthweight?

Infant birthweight is a predictor of future morbidity and mortality, especially for very low birthweight (<1500 g) infants. The risk of dying in the first year of life is estimated to be about 100 times higher for very low birthweight infants than for normal weight infants (2,500 through 3,999 grams or 5.9 to 8.7 pounds). Compared to infants of normal weight, low birthweight infants may be at increased risk of perinatal morbidity, infections, and long-term consequences of impaired development, such as delayed motor and social development or learning disabilities. Mortality risk is lowest for infants born at 3,500-4,500 grams.

What is infant mortality?

Infant mortality occurs when an infant dies in the first year of life. Infant mortality is sub-classified into perinatal, neonatal, and postneonatal mortality. Perinatal mortality includes fetal death of 28 weeks gestation or more and infant deaths in the first 6 days of life. Neonatal mortality is an infant death which occurs in the first 27 days of life. Postneonatal mortality is a death which occurs from 28 days up to a year of life.

What is the sex ratio at birth?

The sex ratio at birth is the ratio of male to female births. The expected sex ratio at birth (male to female) is 1.05

What are the trends in birth outcomes?

The national preterm birth rate has risen 18% between 1990 and 2004 (from 10.6% to 12.5%). Of particular concern is the very preterm rate, about twice as high among non-Hispanic Black infants (4%) as non-Hispanic White (1.6%) and Hispanic births (1.7%).
In Oregon, the 2005 preterm birth rate was 8.2%, up from 5.4% in 2000, an increase of 52%. During that period of time the rate increased 67% among Asians (to 9.2%), 63% among whites (to 8.5%), 37% among Hispanics (to 7.6%), 28% among American Indians and Alaskan natives (to 10.1%), and remained unchanged among Blacks at 9.4%.
Obesity, which has increased nationally approximately 120% since 1980, is associated with the increase in premature births e.g., in extremely obese women, the risk of medically indicated preterm and very preterm births is almost double that for nonobese women. 
Low Birthweight:
Nationally, the percentage of low birthweight infants (<2500 grams, 5 lbs 8 oz) has been increasing steadily over time. In 2005 it was 8.2% of all births, up from 6.7% in 1984. In Oregon the percentage of low birthweight infants was 6.1% in 2005, up from 5.5% in 1995. The Healthy People 2010 goal is 5%.
Nationally, the percentage of infants born with very low birthweight (<1,500 grams, 3 lbs 5 oz) increased from 1.3% in 1990 to 1.5% in 2004. In Oregon the 2005 percentage of infants born with very low birthweight has been stable over the last decade at approximately 1.0%,
Infant Mortality:
National fetal and perinatal mortality rates have declined slowly but steadily from 1990 to 2004. National infant mortality was 686 per 100,000 live births in 2002-2004, slightly down from 692 per 100,000 in 1999-2001. Oregon infant mortality was 570 per 100,000 in 2005, down from 610 per 100,000 in 1995.
National neonatal mortality in 2002-2004 was 461 per 100,000 live born infants with little change from 1999-2001. In Oregon the neonatal death rate was 380 per 100,000 in 2005, up from 320 per 100,000 in 1995. National postneonatal mortality in 2002-2004 was 225 per 100,000 live born infants, slightly lower than the rate of 230 per 100,000 in 1999-2001. in Oregon the postneonatal death rate was 190 per 100,000 in 2005, down from 290 per 100,000 in 1995. The United States continues to rank poorly in international comparisons of infant mortality.
National fertility rates have fluctuated from a high of 3.5 births per woman in the late 1950s, to a low of 1.8 births per woman in the mid-1970s. During the past decade fertility rates have stayed close to the natural replacement rate of about 2.1 births per woman. In Oregon the 2005 fertility rate was 1.9 births per woman, down from 3.6 in 1960. The Oregon fertility rate has been relatively stable since the 1975 low of 1.7 births per woman. According to data from the National Survey of Family Growth, 12% of U.S. couples had impaired fecundity in 2002, up 20% from 1995.
Sex ratio at Birth:
For all races in the United States, the sex ratio decreased from 1.055 in 1970 to a low of 1.046 in 2001. The decrease in sex ratio at birth was found only among Whites in the U.S., and not among African-Americans.

What is the role of environmental exposures in birth outcomes?

Reductions in birthweight or increases in low birthweight have been associated with exposure during pregnancy to lead, solvents, pesticides, polycyclic aromatic hydrocarbons (PAHs), and air pollution. Increases in risk of prematurity or preterm delivery have been related to exposures during pregnancy to air pollution, lead, some solvents, the pesticide DDT, and di-ethylhexyl phthalate (DEHP).
High concentrations of particulate matter (PM10) were associated with 10% increase in early postneonatal mortality in a study of 4 million infants born in the U.S. between 1989 and 1991. The major causes of death associated with PM10 exposure was deaths from respiratory causes and SIDS.
Approximately 10% of problems with fertility are unknown and environmental contaminants including endocrine disruptors have been hypothesized as major contributors. The case of DES revealed environmental contamination can have multi-generational impacts on reproduction that need to be studied and tracked long term.
Numerous studies in various countries throughout the world have reported changes in the ratio of males to females at birth. Although the mechanism which determines the sex of the infant is not completely understood, it has been suggested that environmental hazards can affect how many males are born. Parent and/or the fetus can come in contact with and become exposed to different hazards referred to as endocrine disruptors. Fewer males are conceived when exposure to endocrine disruptors results in a decrease in testosterone. 

More information about environmental exposures and birth outcomes

Centers for Disease Control and Prevention (CDC), Pregnancy Information:
Centers for Disease Control and Prevention (CDC), Tobacco Use and Pregnancy:
Centers for Disease Control and Prevention (CDC), Effects of Workplace Hazards on Female Reproductive Health:
March of Dimes, Environmental Risks, and Pregnancy:
National Environmental Public Health Tracking (NEPHT) Program, Centers for Disease Control and Prevention (CDC):
Oregon Department of Human Services (DHS), Center for Health Statistics: /DHS/ph/chs/
Oregon Environmental Public Health Tracking (EPHT), Department of Human Services (DHS): /DHS/ph/epht/index.shtml
Oregon Health & Sciences University, Global Network for Perinatal and Reproductive Health: