Reproductive Health Indicator:  Growth retardation

Measure: Low birthweight percentage

Birth weight is the first weight of the newborn obtained after birth.  A baby is born with low birth weight when its weight is less than 5.5 lbs (2500 grams) at birth.  A low birth weight baby can be born too small, too early, or both. These conditions often have separate causes. Specific factors may be related to one of these conditions but not the other.

The infant’s birthweight is a predictor of future morbidity and mortality, especially for very low birthweight infants. The risk of dying in the first year of life is estimated to be approximately 100 times higher for very low birthweight infants than for normal weight infants. Compared to babies of normal weight, low birth weight babies may be at increased risk of illness from birth through the first six days of the baby’s life (perinatal morbidity), infections, and the longer-term consequences of impaired development, such as delayed motor and social development or learning disabilities.

The percent of low birthweight births among term singleton births is a useful measure of perinatal health used to examine trends occurring over time and between geographical areas. Low birthweight birth measures also evaluate the effectiveness of existing and new prevention programs. Trends identified by this measure may also reflect the contributions of environmental exposures or other modifiable risk factors that can be the subject of future investigations. 

The primary cause of very low birthweight is premature birth (before 37 weeks gestation). Very low birthweight babies are often born before 30 weeks of pregnancy. Another cause of very low birthweight is intrauterine growth retardation (IUGR). This occurs when a fetus does not grow well because of problems with the placenta, the mother's health or birth defects.

Risk factors associated with very low birthweight include mother’s age (17 years and younger or 35 years and older); exposure to drugs, alcohol or cigarettes during pregnancy; lower socioeconomic status (SES); poor pregnancy nutrition; inadequate prenatal care; and race (e.g., black infants are twice as likely to be very low birthweight as are white infants).

Some studies have reported that environmental factors − including exposure to air pollution, drinking water contaminated with chemical disinfection byproducts and exposure to pesticides, lead, solvents, pesticides, polycyclic aromatic hydrocarbons (PAHs) during pregnancy − can also affect  newborns’ birthweight. However, additional studies are needed to confirm the association of these risk factors with low birthweight.

In Oregon, from 2000 to 2006, low birthweight births were 2.1 percent of live-term singleton births, less than one-third the national rate. As shown in the table, map and graph below, the high of 2.5 percent occurred in 2000 followed by the low of 1.7 percent in 2001; low birthweight births increased to 2.5 percent in 2006, a return to the 2000 level.

The percentage of low birthweight births varied by county from zero percent to a high of 6.5 percent in Baker and Lake counties in 2005 and 2000, respectively. As with other birth statistics, the percentages of low birthweight births tend to be unreliable in rural counties due to small numbers.

The county average percentages of low birthweight vary widely from 1.0 percent to 3.8 percent in Wasco and Lake counties, respectively. However, differences largely disappear when the five most populous counties (Multnomah, Washington, Clackamas, Lane and Marion) are compared to the more rural counties (2.04 percent vs. 2.14 percent, respectively). 

The percentage of low birthweight births does not necessarily inform us about the true health risk associated with low birthweight in a specific community. Consequently, this measure should be interpreted with caution. It should be viewed in conjunction with other reproductive outcome measures such as the infant mortality rate (neonatal and postneonatal), perinatal death rate and morbidity measures that are included on this website as well as with other epidemiological information that may be available for particular populations.

NOTE: Multiple births, including those delivered at term, are usually low birthweight.. Therefore, this measure is restricted to singleton infants born at term and differs from the preterm measures presented in the previous section.

Table 1: Annual percentage and count of live term singleton births that were low birthweight, i.e. less than 5 pounds 8 ounces (< 2500 grams), by county and year. 

Graph 1: Percentage live term singleton births that were low birthweight, i.e. less than 5 pounds 8 ounces (< 2500 grams), by county, summarized for 2000 to 2006. 

Map 1: Percentage of live term births to Oregon resident mothers that were low birthweight. 

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Table 1: Annual percentage and count of live term singleton births that were low birthweight, i.e. less than 5 pounds 8 ounces (< 2500 grams), by county and year.

To protect confidentiality, percentage and counts are not shown in cases where the number of births in the county for the year is less than ten (***). Percentages calculated with less than ten events (count) may be unreliable.

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Graph 1: Percentage live term singleton births that were low birthweight, i.e. less than 5 pounds 8 ounces (< 2500 grams), by county, summarized for 2000 to 2006.

Percentages are shown with their 95% Confidence Intervals (CI). Counties whose entire CI lies above or below the Oregon average (indicated by the dotted vertical line) have percentages that are significantly higher or lower, respectively, than the Oregon average. If the CI includes the Oregon average, the county is considered statistically similar to the Oregon average.

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Map 1: Percentage of live term births to Oregon resident mothers that were low birthweight (<2500 g) , shown in three statistical categories: higher than, similar to, or lower than the Oregon average.

Data are summarized for 2000 to 2006. Statistical significance was determined by comparing the 95% confidence interval of the county rate with the State average.

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