Twins conceived using assisted reproductive technologies (ART) tend to be smaller at birth compared to twins born using natural conception, says a team of researchers in a recent study. And while the growth of ART twins tends to catch-up to their naturally born counterparts by the time both groups reach 2 years old, twins conceived using assisted reproduction remain less physically developed in other ways, the researchers add.¹

It's the first study to compare outcomes of ART versus natural conception using only twins, explained Kristine Anthis, PhD, an assistant professor of Psychology at Southern Connecticut State University in New Haven, and one of the study's co-authors.

Clarifying Inconsistencies
While the use of ART has been increasing rapidly in recent years around the United States, there's been relatively little research in this area, said Anthis and her colleagues. Additionally, the studies that have been published have shown contrasting results. "Research, although sparse, has resulted in inconsistent findings as to the developmental prognosis for infants conceived by assisted reproductive techniques, such as in vitro fertilization and the use of fertility drugs," Anthis and her associates wrote.

To compare the physical development of ART versus naturally conceived twins, Anthis, Lisa Kelly-Vance, PhD, an associate professor of Psychology at the University of Nebraska, and Howard Needelman, MD, a pediatrician at the Munroe-Meyer Institute at the University of Nebraska Medical Center, pulled medical records on 57 sets of twins. Seventy-eight of the twins had been conceived spontaneously, and the other 36 were conceived using assisted reproductive technologies. The researchers also collected data on a smaller group of babies who had reached the age of two to determine their longer-term prognosis.

Developmental Differences Uncovered
The study showed that the average weight of a twin baby conceived through assisted reproduction was about three-quarters that of a naturally conceived baby, on average. While the difference evaporated by the time the ART twins reached 2 years of age, their physical development in certain areas was behind, the study found. These include factors like body control, coordination and fine motor skills. The developmental differences at age two between the two groups was significant, the study investigators noted. More specifically, those conceived using ART were about 15% behind those conceived naturally in those developmental skills.

That data were not expected. "We were surprised with the findings," Anthis recalled.

Unanticipated Outcomes
Past studies also found some differences between twins conceived using ART and those conceived spontaneously. But since more twins are born using assisted reproduction, Anthis and her colleagues thought that the results of those studies were skewed in favor of ART twins. "Some differences between babies of spontaneous conception and assisted conception had been found in past research projects, but they compared apples with oranges," Anthis said.

Thus, Anthis, Kelly-Vance, and Needelman thought that comparing only twins might change the outcome. "We thought that those differences would disappear in this study because we decided only to examine twins," she said. "But there were still physical differences at two years of age between twins who were spontaneously conceived and those whose conception was induced."

No differences in mental development were found between the two groups, Anthis added.

What are the Implications?
So far, it's not known the long-term ramifications of these delayed developmental factors for those children; whether the physical differences found will continue into later childhood and eventually into adulthood is still a question remaining to be answered. More studies to follow these children over longer periods are needed, Anthis said.

She added that her team is "very much interested" in continuing the research in this area.

1. Kelly-Vance L, Anthis KS, Needelman H. Assisted reproduction versus spontaneous conception: a comparison of the developmental outcomes in twins. J Genet Psychol 2004 Jun;165(2):157-67.

John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include coverage of health news for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.