A New York Times article published Saturday notes that a reduction in the number of twins born from fertility treatment could save the health care system $500 million. That's because twins are born prematurely 60 percent of the time and land in the costly neonatal intensive care unit more often than singletons. The article also mentions that these babies are more susceptible to mental retardation and learning disabilities. Just how grave are the risks facing twin babies?
There's a lot of disagreement over the extent of the danger, but we know that twins are five to seven times more likely to die during infancy than singletons. (Triplets, quadruplets, and higher-order multiples are at least 10 times more likely to die.) The chances of a disastrous outcome are still low in absolute terms, however. According to recent research, one twin out of about 27 will die during infancy, compared with one out of 135 among singletons.
Studies also show that twins are more likely to suffer from specific health disorders. While they represent just 2 percent of the general population, twins make up 7.4 percent of the nation's cerebral palsy sufferers. Twins—particularly the second born in a pair—also exhibit respiratory problems at a somewhat higher rate.
Twins may grow up to be just a little bit smaller, on average, than the rest of us. This effect shows up most clearly at birth and is strongly influenced by the fact that twins tend to be born premature. Some researchers argue that these undersize twin babies close the size gap almost completely by age 5. But the prevailing view is that twins never quite catch up to singletons, lagging behind by as much as an inch of height.
If you look across a large population of twins and singletons, the twins may turn out to have slightly lower IQs as well. A compilation of historical studies found an average gap of 4.2 points, although there's plenty of controversy over these results. Recent studies have used more rigorous controls by comparing twins with their singleton siblings. The results are split: In 2005, researchers in Scotland found that twins scored an average of 5.3 points lower at age 7 and six points lower at age 9. However, a similar study conducted in the Netherlands found no difference using a different measure of intelligence.
Premature birth and low birth weight seem to be associated with most of the twins' problems. Twins are born an average of three to four weeks earlier than singletons and tend to be 20 percent lighter and 6 percent shorter. When researchers compare twins and singletons who were born the same size, most of the difference in mortality, respiratory function, and childhood IQ disappeared. (At least that's been the finding of most studies. A few suggest that birth-weight differences cannot fully explain the increased mortality and decreased cognitive functions of twins.) Whatever differences there are between twins and singletons born at the same weight probably have a lot to do with maternal age. Older women are more likely to bear children with certain disorders, and they're also more likely to seek fertility treatments and conceive twins.
Cerebral palsy is the exception to this rule. A number of studies have shown that, even among children of similar birth weight and gestational age, twins are more likely to suffer from the disorder. Researchers suspect that this brain impairment is a result of two fetuses sharing one blood supply, often unequally. Also, in certain cases when one twin dies in utero, the survivor is more than 100 times more likely to have cerebral palsy.
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Explainer thanks Eric Shinwell of Kaplan Medical Center.