Low birthweight (LBW) represents multiple factors: infant current and future morbidity, as well as premature mortality risk, and maternal exposure to health risks. LBW serves as a predictor of premature mortality and/or morbidity. LBW children have greater developmental and growth problems, are at higher risk of cardiovascular disease later in life, have a greater rate of respiratory conditions, and have higher rates of cognitive problems such as cerebral palsy, and visual, auditory, and intellectual impairments.[2-5]
From the perspective of maternal health outcomes, LBW indicates maternal exposure to health risks in all categories of health factors, including her health behaviors, access to health care, the social and economic environment the mother inhabits, and environmental risks to which she is exposed. Authors have found that modifiable maternal health behaviors, including nutrition and weight gain, smoking, and alcohol and substance use or abuse can result in LBW.
Low Birthweight is a Percentage
Low Birthweight is the percentage of live births where the infant weighed less than 2,500 grams (approximately 5 lbs., 8 oz.).
Births are Counted in the Mother’s County of Residence
Births are counted in the county corresponding to the mother’s address on the child’s birth certificate, not the county the child was born in.
Some Data are Suppressed
A missing value is reported for counties with fewer than 10 low birthweight births in the time frame.
The number of live births for which the infant weighed less than 2,500 grams (approximately 5 lbs., 8 oz.) over seven years.
Total number of live births for which weight was recorded over seven years.
This measure can be used to track progress with some caveats. It is important to note that the estimate provided in the County Health Rankings is a 7-year average. However, in most counties, it is relatively simple to obtain single year estimates from the resource included below.
Years of Data Used
National Center for Health Statistics - Natality files
Data on deaths and births were provided by NCHS and drawn from the National Vital Statistics System (NVSS). These data are submitted to the NVSS by the vital registration systems operated in the jurisdictions legally responsible for registering vital events (i.e., births, deaths, marriages, divorces, and fetal deaths). We requested this data for the first time for the 2018 Rankings. This was done because of the discontinuation of Health Indicators Warehouse. This change also allows to to perform additional analyses for state and national reports which if obtained from CDC WONDER would have numerous missing counties.
Counties can find the same data from CDC WONDER. However, we use the raw data files. CDC WONDER does not report data for all counties per their missing data criteria.
The methods for calculating the error associated with death rates can be found here:
For counties with fewer than 20 births a missing value for all values is reported.
For counties with between 20 and 99 births a gamma adjustment from the poisson distribution is used to calculate the CIs (see page 87: http://www.nber.org/mortality/2002/docs/techap99.pdf).
For counties with 100 births or more CIs are calulated according to the normal distribution. Standard errors (SE) and birth rates for each age group are calculated. These SEs are squared and multiplied by the square of the weights and then divided by the total number of births over all age groups. The sum of these provides the variance of the estimate for each county. The square root of the variance gives the standard deviation which is then used as estimate +/- 1.96*STDEV.
 Paneth NS. The problem of low birth weight. Future Child. 1995;5:19-34.
 Knoches AML, Doyle LW. Long-term outcome of infants born preterm. Baillieres Clin Obstet Gynaecol.1993;7:633-651.
 Hack M, Klein NK, Taylor HG. Long-term developmental outcomes of low birth weight infants. Future Child.1995;5:176-196.
 Irving RJ, Belton NR, Elton RA, Walker BR. Adult cardiovascular risk factors in premature babies. Lancet.2000;355:2135-2136.
 Shenkin SD, Starr JM, Deary IJ. Birth weight and cognitive ability in childhood: A systematic review. Psychol Bull.130:989-1013.
 Bailey BA, Byrom AR. Factors predicting birth weight in a low-risk sample: The role of modifiable pregnancy health behaviors. Matern Child Health J. 2007;11:173-179.