Two population-based studies, conducted at the University of Florida ( UF ), found that twins, triplets and other multiples have a nearly 50 percent greater chance of being born with birth defects, and boys tend to be more at risk than girls.
UF researchers who studied all Florida births from 1996 through 2000 found multiples have a higher risk than babies born singly of developing 23 of 40 birth defects, such as spina bifida.
The same team of researchers, from UF's Maternal Child Health Education Research and Data Center, studied 4,768 pairs of opposite-sex twins and found that boys had a 29 percent higher risk for birth defects than girls. This could be because boys tend to develop at a slower pace, leaving a little more time for potential problems to arise.
" In the past 20 years, multiple births have increased because of greater reliance on assistive reproductive technology, especially among women delaying childbirth until their 30s and 40s," said Yiwei Tang, a lead researcher on both studies. " In offering these options to women, full disclosure of an increased risk of birth defects should be made."
Multiples had the highest risks of having certain brain, heart, bladder and liver defects
Although the risks are greater for multiple-birth babies, the number of children born with birth defects is still small. About 3.5 percent of multiples are born with birth defects, whereas 2.5 percent of single-birth babies are, the research shows.
The team analyzed years of data from Florida Birth Vital Statistics and the Florida Birth Defects Registry, studying 972,694 births for the multiple-birth study. Of those, about 28,000 were multiples, about 3 percent of all births.
Among twins, boys were twice as likely as their sisters to have defects of the genital and urinary organs and five times as likely to be born with an obstruction between the stomach and small intestine. But congenital hip dislocation was 10 times more common among girls.
Previous research has shown that boys tend to be more susceptible to birth defects, but little was known about defects among opposite-sex twins, who develop in the same environment and have similar risks for genetic defects, said Christine Cronk, at the Medical College of Wisconsin.
Because older mothers naturally have an increased risk of giving birth to children with birth defects, the researchers used statistical models to factor out age, race and even education levels that could have led to inaccurate results, Tang said.
Source: University of Florida, 2005