While it generally has been considered desirable for premature infants to catch up in size with normal-weight infants, studies also have linked unusually rapid growth in childhood with an increased risk of obesity, heart disease and diabetes later in life.
Most of the catch-up growth in girls studied appeared to occur from ages 8 through 20. And while premature boys were half as likely as normal-birthweight boys to be obese, obesity rates were similar among premature and normal-birthweight girls.
"On the surface it appears that the male very low birthweight subjects might be at a disadvantage," Dr. Maureen Hack and colleagues said. "However, we are more concerned about the future health of the very low birthweight females."
While it's too soon to tell what health problems the young women will face later on, Hack said the findings suggest parents of girls born prematurely should take special care to make sure they have a healthy diet and avoid becoming overweight.
The study appears in the July issue of Pediatrics, published Monday.
The research involved 103 boys and 92 girls born in the late 1970s at Rainbow Babies and Children's Hospital in Cleveland, Ohio, where Hack is a prominent prematurity researcher.
On average, the infants were born about 10 weeks early weighing less than three pounds. They were followed through age 20 and compared in growth with 208 normal-weight youngsters.
Prematurely born girls and normal birthweight girls were both about 5 feet 3 inches (1.60 at age 20. Girls born prematurely weighed on average about 143 pounds (64.35 at that age, six pounds (2.7 less than normal-birthweight girls.
In contrast, the premature boys were 5 feet 7 inches (1.70 and 152 pounds (68.4 at age 20 — one inch (2.5 centi shorter and 24 pounds (10.8 lighter than their normal-birthweight counterparts.
Only 7 percent of the premature boys were obese at age 20, about half as many as the normal-birthweight boys. But obesity rates were more similar among premature and normal-birthweight girls — 15 percent and 18 percent respectively.
The prematurely born boys were generally sicker babies than the premature girls, echoing previous prematurity research, though significant prematurity complications including respiratory distress also occurred in the girls.
"We thought that neither of them would catch up," Hack said, so "for the males, it's not that surprising" that they were significantly smaller at age 20 than their peers, Hack said.
The effects of female hormones that kick in when girls reach puberty may partly explain their catch-up growth, Hack said.
Dr. Saroj Saigal, a prematurity researcher at McMaster University in Hamilton, Ontario, said the study echoes previous findings on gender growth differences in adolescents but is one of the few to follow premature infants into adulthood.
"That this catch-up may not be a real advantage" for girls is only speculative, and needs to be proven with more follow-up studies, Saigal said.
It may be that rapid growth during puberty and adolescence is the most problematic, Saigal said.
"Perhaps we should strive for continuing weight gain and catch-up earlier on," when premature babies are still hospitalized, she said. "We're still not able to achieve that."
By Lindsey Tanner