Birth weight has significant and lasting effects, a new study finds. Weighing less than 5.5 pounds at birth increases the probability of dropping out of high school by one-third, reduces yearly earnings by about 15 percent and burdens people in their 30s and 40s with the health of someone who is 12 years older.
The study, presented May 22 in Washington, D.C. at the National Summit on America's Children, is the first to link birth weight with adult health and socioeconomic success using a full, nationally representative sample of the U.S. population. It is based on an analysis of more than 35 years of data on more than 12,000 individuals from the Panel Study of Income Dynamics, conducted since 1968 by the University of Michigan Institute for Social Research (ISR).
Funded by the National Institute on Aging, the analysis includes data from the original study families, plus their descendants who have gone on to form families of their own. Because of the study's unique genealogical design, the researchers were able to compare outcomes for siblings to isolate the impact of low birth weight apart from other common family conditions siblings share.
According to the authors, economists Rucker Johnson at the University of California, Berkeley, and Robert Schoeni at U-M, the study provides the most detailed look to date at how well-being and disadvantage are transmitted across generations within families.
"The poor economic status of parents at the time of pregnancy leads to worse birth outcomes for their children," Johnson and Schoeni write in a working paper from the U-M National Poverty Center. "In turn, these negative birth outcomes have harmful effects on the children's cognitive development, health, and human capital accumulation, and also health and economic status in adulthood. These effects then get passed on to the subsequent generation when the children, who are now adults, have their own children."
Among the key findings:
• Compared to their normal birth weight siblings, low birth-weight children are 30 percent less likely to be in excellent or very good health in childhood. They also score significantly lower on reading, passage comprehension, and math achievement tests. Low birth-weight children are roughly one-third more likely to drop out of high school relative other children.
• Low birth weight has significant negative effects on adult health, equivalent to being 12 years older in one's 30s and 40s. Weighing less than 5.5 pounds at birth increases the probability of being in fair or poor health as an adult by over 70 percent. Not only does birth weight have large and lasting effects across the life course, the researchers note, but its effects become larger later in life. For example, low birth weight children are nearly twice as likely as their normal birth-weight siblings to be in problematic health by ages 37-52 (23 percent versus 12 percent).