The high cost of having some babies ... gets higher by the numbers.

Roughly 8.5 percent of U.S. married couples are infertile. Though that rate has not increased in recent years, the number of people who sought treatment between 1968 and 1982 nearly tripled, fueled in large measure by increasingly successful in vitro (test-tube) fertilization techniques.

But these treatments are expensive, as two studies document in the July 28 New England Journal of Medicine.

The first studied the 1992 costs of a successful delivery of one or more babies following in vitro fertilization. Many couples undergo several cycles of egg harvesting, test-tube fertilization, and subsequent embryo implantation before a baby is born. The less fertile the couple, the more cycles they tend to require before giving birth. Overall, less than half of couples treated for infertility succeed in giving birth.

In a new analysis, Peter J. Neumann of the Project HOPE Center for Health Affairs in Bethesda, Md., and his colleagues find that the costs of having a baby climb from about $67,000 per delivery for couples undergoing one fertilization cycle to 114,000 per delivery after six cycles.

That's just the average cost. Certain forms of infertility can jack up the charges dramatically For instance, when the mother is 40 or more years old and the father infertile, delivery costs climb from about $160,000 for one cycle of treatment to $800,000 for couples requiring six cycles.

Much of the high economic cost associated with in vitro fertilization traces to the higher proportion of multiple births that occur among treated couples, Neumann and his collaborators calculate. The reason? Multiple births are usually premature, with smaller and sicker babies who require more intensive -- and expensive -- care.

. . . gets higher by the numbers

A second study in the New England Journal of Medicine shows why hospital costs multiply for each additional child. Among 13,206 pregnant women who delivered at Boston's Brigham and Women's Hospital between 1986 and 1991, single babies remained hospitalized 4.6 days on average; twins typically stayed 8.2 days. Births involving more babies involved even longer stays. Moreover, 15 percent of single babies needed intensive care, compared to roughly half the twins and three-quarters of triplets, quadruplets, and quintuplets, say Tamara L. Callahan of Massachusetts General Hospital and her coworkers. Multiple births also increased the mother's need for care.

In the end, after accounting for a mother's age, race, and type of insurance, Callahan's group found that hospital charges for a single baby totaled about $9,850. By contrast, twins incurred average charges of $37,950 ($18,975 per baby), and the birth of triplets cost $109,765 ($36,588 per baby).

One-third of the twins and three-quarters of the triplets in the study were born to couples treated for infertility. Indeed, Callahan's team estimates that if each multiple pregnancy had involved a single baby, "the [annual health care) savings would have been more than $3 million in this one hospital" And as they did not factor in the additional postdischarge costs typically associated with twins and triplets, these figures probably understate the total costs of multiple births.

With health care funds tight, these figures raise several ethical issues, the researchers say. For example, who should have access to expensive fertility treatments? Who should pay the bill? Moreover, because not all fertility treatments run the same risk of fostering multiple births, should physicians focus on those most likely to result in a single baby?