New mothers are staying in the hospital a little longer, about half a day more on average, according to a first study since a public outcry over "drive-by deliveries." The trend began even before a federal law requiring insurance coverage of 48-hour stays went into effect.

In the early 1990s, many insurance companies began paying only for 24-hour hospital stays for vaginal childbirth.

Doctors say that often is long enough for healthy women with uncomplicated vaginal deliveries who are having their second baby, but that first-time mothers who generally have longer labors and more questions about newborn care often need a second day.

In the wake of complaints about inadequate care, states began passing laws in 1995 and 1996 requiring insurers to pay for 48-hour stays. A similar federal law was passed in 1996 and took effect last year.

In 1980, the nation's average stay for a vaginal delivery was 3.2 days. That dropped to 1.7 days by 1995, but inched up to 2.1 days by 1997, the Centers for Disease Control and Prevention reported yesterday.

In 1995, 1.4 million new mothers -- 37 percent -- were hospitalized for one day or less. In 1997, 951,000 new mothers -- 25 percent -- had such short stays, the CDC reported.

Separate data from the Aetna-U.S. Healthcare insurance company show its members' maternity stays after vaginal delivery increased from 1.7 days in 1997 to 2.4 days in 1998.