Fertility clinics are overusing a laboratory technique and costing infertile couples and some insurers hundreds of extra dollars, a new study suggests.

At issue is a procedure that injects a single sperm into an egg. The method is considered the best option for couples in which the man has defective sperm or extremely low sperm counts.

But many clinics are using it for other infertile couples, even though it often doesn't work as well as the standard lab dish method, according to a study in Thursday's New England Journal of Medicine.

Sperm injection adds about $1,500 to the $12,400 average cost of an in vitro fertilization treatment cycle, the authors said.

"This paper is particularly troubling because we've got a major shift in practice that isn't evidence driven. The paper suggests it may be driven by money," said Arthur Caplan, director of the University of Pennsylvania's Center for Bioethics.

The study's lead author, Dr. Tarun Jain of the University of Illinois at Chicago, said more research is needed and he couldn't be certain money explains the findings. But the study does indicate routine use of the technique does not benefit many couples, he added.

Dr. Steve Ory, president of the American Society of Reproductive Medicine, said there are many reasons a clinic might use sperm injection to treat an infertile couple, and it's not clear that the technique is overused.

According to the researchers and Ory, some clinics may use it as in cases where there's only a few eggs available, or in cases where the standard method failed. Some may opt to use it for all couples, believing it can increase the chance of success.

"There's a lot of art to this," Ory said.

The research team reviewed a decade of results that hundreds of fertility clinics reported to the federal government. In 2004, about 58 percent of treatment attempts included sperm injection — up from 11 percent in 1995.

But the proportion of couples who have trouble conceiving because of the man's sperm has stayed constant, at around 34 percent. This suggests that the sperm-injection technique is being urged on many couples who do not need it and might be better off with traditional lab dish, or in vitro, fertilization, Caplan said.

In the standard method, eggs are placed in a petri dish with 100,000 to 500,000 sperm in the hope that one will wiggle through an egg's outer membrane and fertilize it. The sperm-injection technique leaves nothing to chance, using a fine glass needle to shoot a sperm inside the egg.

Sperm injection does not increase overall success rates for healthy births. The researchers found that among infertility treatment attempts with successful egg retrievals in 2004, about 31 percent of those involving sperm injection resulted in a live birth. The percentage was higher — 33 percent — for those that did not use the sperm injection.

Perhaps nature is better than doctors at selecting the right sperm to produce a healthy baby, Jain said.

Another problem: A higher risk of birth defects is seen in children produced through sperm injection, although scientists haven't sorted out if that's a result of the technique itself or abnormalities in the fathers' sperm, the authors noted.

They also noted that sperm-injection rates were higher in three states — Illinois, Massachusetts and Rhode Island — that mandate coverage of the technique than in states without such a requirement.

Jain noted he didn't have specific data about each couple or each treatment attempt, so it was hard to determine whether, for example, clinics that did a lot of sperm injection also had disproportionate numbers of patients with more severe cases.

Also, despite a federal requirement that fertility clinics report verifiable data, not all comply, he noted. In 2004, there was no or incomplete data available for 11 percent of clinics.