The first decade of the 2000s saw a small but significant increase in the survival of extremely premature infants without early neurodevelopmental problems, according to a study of nearly 4,500 babies born at 11 U.S. medical centers.
"We've seen a slight increase in the number of infants who survived without signs of impairment at 2 years of age," chief author Dr. Noelle Younge of Duke University in Durham, North Carolina, told Reuters Health.
For the study, published in the New England Journal of Medicine, Younge and her colleagues looked at survival rates, with and without disability, for these extreme preemies over three periods - from 2000 to 2003, from 2004 to 2007 and from 2008 to 2011.
A typical pregnancy lasts about 40 weeks, and babies born before 37 weeks are considered preterm. Those born much earlier, at around 22 to 24 weeks gestation, are considered to be at the limit of viability and likely to have serious neurological and other problems.
Understanding the likelihood of survival and the odds for disabilities can help doctors counsel parents of extreme preemies about their child's prognosis, the study team writes.
Younge's team found that among all children born at 22 to 24 weeks, rates of survival without neurodevelopmental impairment - which was assessed at age 18 to 22 months of corrected age - increased by 4 percentage points from the earliest study period to the most recent.
But the actual odds were small at the start and remained small, with impairment-free survival going from 16 percent to 20 percent.
The death rate declined from 70 percent in the first period to 64 percent in the most recent period.
The real improvements were seen in children born at 23 weeks and 24 weeks. The impairment-free rates went from 7 percent to 13 percent in the 23-week group and from 28 percent to 32 percent in the 24-week group.
The researchers said lower rates of infection and the greater use of steroids given to mothers to help the fetus mature faster may be responsible for the improvements.
But those interventions have their limits.
There was no comparable improvement over the 12-year study period for babies born at 22 weeks gestation. Only 1 percent survived without neurodevelopmental impairment.
The raw survival rates showed a similar pattern, with most of the improvement reflecting higher survival rates among infants born in the 24th week.
In that group, the rates went from 49 percent in the earliest period to 56 percent in the most recent one.
In comparison, in 2008-2011, survival rates were 24 percent if the baby made it to 23 weeks gestation compared to just 3 percent if the baby was born at 22 weeks.
"With the 22-weekers we didn't see a lot of difference," said Younge, a neonatologist and assistant professor of pediatrics at Duke. "At 23 weeks we did see an improvement over time."
That seems to be the key demarcation point because "between 22 and 23 weeks, the lungs are really maturing. The brain is maturing. So in this 22-week window certain infants have met maturation milestones and can survive," she said.
The researchers caution that "there is likely to be substantial variation" in the long-term functioning of children classified as having neurodevelopmental impairment in early childhood.
"Although early neurodevelopmental assessment is important for the timely identification of children at risk for long-term neurologic impairment or developmental delay, its capacity to predict later functioning is limited," the study team writes. "Many children will catch up to their peers by school age, whereas other children will have persistent impairment. Conversely, some children without signs of neurodevelopmental impairment in early childhood will have impairments that manifest at school age."