Infant mortality is at its lowest rate ever. Now fewer than three percent of babies worldwide die within the first five weeks of life, which is surely cause for celebration.

Many of the infants who have been saved, however, did not enter this world easily. A new analysis published online Thursday in The Lancet found that babies who were pre-term, delivered to mothers with infectious diseases, or had other complications had a 39 percent chance of having at least one type of cognitive or physical impairment.

The new analysis examined data from 153 previous studies to find data on 22,161 babies who had survived a complicated pregnancy or birth. Of the 39 percent of those children who suffered some impairment from the birthing process, 59 percent ended up with learning difficulties or developmental delays and 21 percent had cerebral palsy. "These impairments cause a major socioeconomic burden, especially in resource-poor countries," the researchers note.

The problem isn’t getting better. In the past four and a half decades, the frequency of long-term impairments for newborns who survive complications has remained about the same, according to the research team, which was led by Michael Mwaniki of Kenya Medical Research Institute's Center for Geographic Medicine Research. The scientists suggest that improved medical technology has "increased survival in neonates who would have otherwise died," which cancels out advances in treatment for less severe cases.

Even in wealthy countries where health care is widely available, complications during pregnancy increase the odds a child will have a chronic disease later in life. In middle- and lower-income countries, impairment of a child can mean "major burdens on families and societies, and shortened life expectancy," the authors write.

But many of the tools, such as antenatal steroids, for improving the outlook for these children already exist. "Complications of preterm birth can be reduced by cost-effective interventions," as can those from birthing trauma, according to the authors. Additional long-term support and rehabilitation can also help.