More than 40 percent of unmarried American teens have had sex, according to a new study.

The study from the U.S. National Center for Health Statistics reports that 4.3 million teens have had sex at least once.

The trend has flattened during the last two decades when teen pregnancy was in a decline.

"One of the great success stories of the past two decades has been the extraordinary declines in teen pregnancy and childbearing," Bill Albert, chief program officer at the National Campaign to Prevent Teen and Unplanned Pregnancy, told U.S. News and World Report. "This progress has recently stalled out."

Even more surprising, 1 in 5 teen girls and 1 in 4 teen boys said they would actually be pleased if pregnancy resulted from the act.

"This is really quite alarming," Albert said. "I don't think it takes a Ph.D. to understand that in this day and age and in this economy the route to success doesn't begin with a family at age 16."

The report also found that about one quarter of female teens and 29 percent of males claimed to have had multiple partners.