While there have been calls to lower the legal drinking age from 21 in the United States, a new study suggests such a move could result in more unplanned pregnancies and premature births among young women.

Researchers found that in the late-1970s and 1980s - when U.S. states varied in their minimum drinking ages - there was a connection between more lenient drinking ages and the risk of premature birth and low birthweight among women younger than 21.

There was also evidence linking a legal drinking age of 18 to a higher rate of unplanned pregnancies among young women. It’s this trend that may account for much of the risk of premature delivery and low birthweight, the investigators report in the Journal of Health Economics.

Unplanned pregnancies are associated with a higher rate of complications, at least partly because women are less likely to get proper prenatal care.

The findings do not prove that lower legal drinking ages lead to more unplanned pregnancies and birth complications, but do show an association between the two.

“It is always possible that other factors besides drinking age laws are causing the effects we see, but we don’t see similar birth weight and prematurity effects for women over 21,” researcher Tara Watson, of Williams College in Williamstown, Massachusetts, told Reuters Health.

In the early-1970s, 29 U.S. states lowered their legal drinking age to 18, 19 or 20. But after a subsequent rise in drunk-driving accidents among young drivers, many states began to reverse course. A change in federal law eventually pushed all states to adopt a minimum drinking age of 21 by 1988.

However, the benefits of the age-21 law are still debated. Last year, a group of more than 100 U.S. university presidents and chancellors known as the Amethyst Initiative called for a reevaluation of the legal drinking age - citing a “clandestine” culture of binge-drinking among college students as one reason that the age-21 law is not working.

For the current study, Watson and colleague Angela Fertig, of the University of Georgia in Athens, analyzed birth records and government survey data on alcohol use for the years 1978 to 1988.

They found that a legal drinking age of 18 was associated with a 21-percent increase in prenatal drinking among 18- to 20-year-old women. It was also linked to a 6-percent increase in the odds of low birthweight and a 5-percent rise in the risk of preterm delivery.

The researchers also found evidence linking these effects to a higher rate of unplanned pregnancies in states with a minimum drinking age of 18. Young African-American women in these states were 25 percent more likely to have no paternal information on their child’s birth certificate - which can be considered a proxy for unplanned pregnancy.

According to Watson and Fertig, the findings suggest that in the debate over the legal drinking age, the potential effects on pregnancy outcomes should be considered.

“We find that the minimum drinking age law has important unintended benefits which should be considered before lowering the drinking age,” Watson said.

SOURCE: Journal of Health Economics, May 2009.