April 18, 2006 -- Babies born to women who get pregnant very quickly after having a child or wait many years between children are at greater risk for potentially serious delivery-related complications, including premature birth and low birth weight.
The analysis of studies involving more than 11 million women is published in tomorrow's Journal of the American Medical Association.
Researchers found that infants born to women who conceived less than six months after giving birth had a 40% increased risk for being born prematurely and a 61% increased risk of low birth weight, compared with infants born to mothers who waited 18 months to two years between pregnancies.
Babies whose mothers had their previous child at least five years earlier had a 20% to 43% greater risk for being born prematurely, having a low birth weight, or being small for their gestational age.
Although timing may not be everything, the analysis suggests that betterspacing could have a dramatic impact on neonatal complications and deaths, both worldwide and in the United States.
4 Million Deaths
Four million babies die each year within a month of birth, including almost 19,000 in the U.S. The World Health Organization estimates that just over a fourth of these deaths are directly related to premature birth.
"Little changes that increase the time between pregnancies could have a huge public health impact," epidemiologist Rachel Royce, PhD, tells WebMD.
She adds that in the United States between 6% and 10% of pregnancies among women who have already given birth occur less than six months after childbirth.
In the newly published analysis, researcher Agustin Conde-Agudelo, MD, MPH, and colleagues analyzed 67 studies published between 1966 and 2006. All of the studies included in the analysis attempted to account for known influences on pregnancy outcomes, including maternal age and socioeconomic status.
The risk for preterm birth, low birth weight, and small size for gestational age increased by 1.9%, 3.3%, and 1.5%, respectively, each month that the time between pregnancies was shortened from 18 months.
For each month between pregnancies longer than five years, the risk for these adverse outcomes increased by 0.6% to 0.9%.
"After birth, the recommended interval to next pregnancy is at least 18 months and no longer than 60 months in order to reduce the risk of adverse [birth] outcomes," Conde-Agudelo tells WebMD.
Breastfeeding and Birth Control
Based on this analysis and his own studies, Conde-Agudelo says he's now convinced that birth spacing is the most effective intervention available for preventing deaths among newborns.
In an editorial accompanying the study, Royce writes that interventions to increase the time between pregnancies, such as encouraging breastfeeding and improving access to birth control, could reduce infant deaths.
Breastfeeding is a natural, although not infallible, form of birth control. So women who follow recommendations to breastfeed exclusively for at least the first six months of her baby's life would be both optimizing the she provides that baby and lowering her risk of having an adverse outcome the next time she gives birth.
New moms may also want to continue taking their vitamins after giving birth to reduce her risk. It has been suggested that depletion of maternal stores of folate during and after pregnancy may be responsible for poor outcomes.
This theory was bolstered by one study, which found that neural tube defects (such as spina bifida) were more likely to occur among children conceived within six months of a previous live birth, compared with those conceived after one to two years.
It is less clear, Royce says, why longer intervals between pregnancies would be associated with increased risk.
"While longer intervals are likely to be beyond personal control, pregnancies occurring after an interval longer than 60 months may require more careful monitoring to avoid [unwanted] outcomes," she writes.