Over the last 15 years, methamphetamine abuse has become the most common reason for pregnant women to seek drug counseling, a new U.S. study shows.

In 1994, the study found, 8 percent of pregnant women who entered drug treatment programs were there for methamphetamine abuse. By 2006, that figure had increased to 24 percent.

"Methamphetamine has become the primary substance compelling treatment during pregnancy," Dr. Mishka Terplan and colleagues at the University of Chicago report in the journal Obstetrics & Gynecology.

The finding, the researchers point out, highlights a need for more effective drug abuse screening during prenatal care.

Many women in their study came to treatment through the criminal justice system, which raises the possibility that drug and alcohol screening is often taking place in the justice system rather than during prenatal care, the researchers note.

The findings are based on a government database of admissions to federally funded substance-abuse treatment centers, which captures more than 80 percent of such admissions each year in the U.S.

In 1994, Terplan's team found, methamphetamine was one of least common reasons for pregnant women to enter drug rehab — accounting for fewer than 1,500 admissions and trailing behind cocaine, alcohol and opiates.

By 2006, however, methamphetamine had become the most common reason for pregnant women's admissions, accounting for more than 5,300.

In 1994, three quarters of pregnant women treated for methamphetamine abuse were white, and nearly all lived in Western U.S. states. By 2006, the problem had spread to the South and Midwest, and also increased among Hispanic women.

In the context of the 4 million births each year in the U.S., the researchers note, few pregnant women seek treatment for methamphetamine abuse.

"Yet," they add, "these patterns raise concerns for both women's life course and for psychosocial development of children."

Little is known about the effects of methamphetamine on pregnancy and fetal development. Research suggests that the drug raises the risk of low birthweight, and one recent study of young children found that prenatal methamphetamine exposure appeared to alter the structural development of the brain.