The amazing progress of medical science over the last two decades has resulted in millions of lives saved as well as created. It has also produced unanticipated consequences.

Nowhere is that more apparent than in the earliest moments of life.

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In fertility labs across the country, couples who want to have babies are being assisted with an array of reproductive drugs and procedures. Those advances have extended the biology-imposed age limits of motherhood well into the late 30s and early 40s and brought the joys of parenthood to millions of childless couples.

At the same time, advances in neonatal technology — respirators that do less damage to tiny, still-forming lungs, and drugs that do a better job of preventing infection — have significantly reduced the number of babies who die in their first year of life. From 1990 until 2005, the nation's infant mortality rate declined roughly 20 percent, an important achievement in medical science.

But in that same time frame, the rate of premature births increased at roughly the same 20 percent rate, a trend line that has had profound long-term consequences in public health policy and costs.

We know that the egg-producing drugs given to women going through fertility treatment result in a much higher rate of multiple births. We know that women who are carrying twins and triplets are much more likely to deliver them early, often at less than 38 weeks gestation. And we know that the older a mother is, the more likely she will deliver a baby prematurely.

While all those trends have had an impact, experts believe that other factors have also played a role in the increase in premature birth rates. Whatever the cause, neonatal intensive care units at Grady Memorial Hospital in Atlanta and other hospitals around the country are seeing a discouraging number of premature babies. Most preemies grow up to have good health. But the known consequences of prematurity — chronic lung problems, cerebral palsy, mental retardation and learning disabilities — are well-documented. The smaller the baby, the more significant the risk of problems.

But bigger and longer-term studies of premature infants are producing even more surprising and unanticipated results, including a significantly higher risk of death during childhood and a higher incidence of infertility in adulthood.

For boys born between 22 and 27 weeks gestation, the risk of death between ages 1 and 6 was five times higher than for full-term newborns and seven times higher between ages 7 and 13, a study of 1.2 million infants born between 1967 and 1988 shows. For boys born between 28 and 32 weeks, the risk of death during childhood was 2.3 to 2.5 times higher.

The most prematurely born girls had nearly 10 times the normal risk of death in early childhood, but they had no significant increased risk beyond age 6. Investigators suggested that a higher incidence of childhood cancers and congenital abnormalities may have raised the risk of death.

The lower rate of reproduction was harder to explain, but prematurely born boys were 76 percent less likely to have children and premature girls 67 percent less likely to conceive when they reached adulthood. Pre-term women were also at increased risk of having premature babies of their own, researchers said.

The huge study, released just this week, involved babies born in Norway, which consistently has a better record of health outcomes than most countries. In the United States, the effort to reduce premature births has been plagued with persistent racial and income disparities. One in eight American infants — more than a half million a year — is born too soon.

The good news is that more of them survive than ever. The more discouraging news is that we aren't even close to solving the real problem.

• Mike King is a member of the editorial board. His column appears Thursdays.