How Does Birth Weight Affect
Adult Health And Success?
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
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).
• The earnings penalty for being born low weight also increases
with age, from 10.2 percent at age 25 to 15.6 percent at age 35.
Low birth weight is linked to a 10 percent reduction in hourly
wages from ages 18-26, compared to the wages of normal birth-weight
siblings, but a 22 percent reduction in wages from ages 37-52.
Low birth-weight children, relative to their normal birth-weight
siblings, work 7.4 percent fewer hours in adulthood. These effects
of poor infant health persist, in sibling comparisons, after accounting
for the independent effects of birth order, mother's age at birth,
birth year cohort, race/ethnicity, family structure, parental
income, and parental fertility timing.
• Not only does low income and lack of health insurance
during pregnancy increase the likelihood of poor birth outcomes,
but limited parental resources also influence the lasting impacts
of poor infant health. The absence of health insurance during
childhood intensifies the negative impact of low birth weight.
For example, the harmful effects of low birth-weight on adult
labor force participation is over twice as large if the adult
did not have health insurance in childhood. Additionally, the
harmful effects of low birth weight on adult health are 2.7 times
larger for those who were uninsured in childhood.
• Among the poorest families (those with incomes of less
than $15,000 in 1997 dollars), increasing income by $10,000
lowers the probability of low-weight birth by 2.18 percent. Increasing
income by the same amount among lower middle income and high-income
families does not significantly influence birth weight. Moreover,
while parental income during pregnancy is beneficial at the low
end of the income distribution for all newborns, the income effect
is much larger for infants who were predisposed to be born low
weight because their mothers were themselves born low weight.
• The large racial differences in adult health status through
mid-life in the U.S. can be fully explained by a few early life
factors—birth weight, parental family income and health
The ISR Panel Study on Income Dynamics is funded primarily by
the National Science Foundation, the National Institute on Aging,
and the National Institute on Child and Human Development. The
study is co-directed by Schoeni and U-M economist Frank Stafford.