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, according to results recently published online in the Maternal and Child Health Journal.
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, according to findings published this month in Birth Defects Research (Part A): Clinical and Molecular Teratology.
"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, M.D., an assistant professor of pediatrics and 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.
"Though birth defects are not a common occurrence, when they do occur within a family, it can be life-altering," said Jeffrey Roth, Ph.D., an associate professor of pediatrics, director of the data center and a study co-author. "For the affected family, it doesn't matter that what has happened to them is a rare event."
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.
"The strength of population-based research is that all women in Florida who gave birth during this time period were taken into account," Roth said.
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, Sc.D., an associate professor and leader of the birth defects epidemiology team at the Medical College of Wisconsin.
"(The UF researchers have) speculated that certain birth defects, which occur very early in gestation, have to do with developmental speed in males and females," Cronk said. "The assumption is that if (the baby) is further ahead in development, that would serve as protection against defects."
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. This way, they only compared babies born to similar mothers, Tang said.
This information allows prospective parents, especially those considering fertility treatments that will increase their odds of having multiple children, to make more informed choices, said Michael Resnick, Ed.D., a pediatrics professor and a co-author of the study.
Informed decisions also increase the chances that parents and their physicians will be prepared to provide the best care should children be born with birth defects, Resnick said.
"This study adds to the knowledge to help genetic counselors, psychologists and physicians to better prepare parents as to what the outcomes might be," Resnick said. "They can discuss this information and ask questions before deciding to start a family."