Nearly 13 percent of parents reported usually sharing their bed with their baby in 2000, up from 5.5 percent in 1993, the study says.
Almost half of parents said their infant spent at least some time sleeping on an adult bed during the previous two weeks, and 20 percent of parents said their infant slept with them in an adult bed more than half the time.
The greatest increase was among white mothers, a group that has historically lower rates of sleeping with their babies than other races or ethnic groups, says Marian Willinger, lead author of the study and a researcher in the pregnancy and perinatology branch of the National Institute of Child Health and Human Development.
"I was surprised at the prevalence of routine bed sharing, though not as surprised by the numbers of infants who spent some time sleeping in an adult bed," Willinger says. "As a parent, you know babies do end up in your bed."
The study appears in the January issue of the Archives of Pediatrics and Adolescent Medicine.
Despite the increasing popularity, bed-sharing remains controversial. Those opposed to the practice, including the American Academy of Pediatrics, warn that sleeping with adults puts infants at risk of suffocating under tangled sheets and heavy quilts, of falling off the bed, or of becoming wedged between heavy furniture or between the headboard and the bed.
Supporters of bed-sharing feel even more strongly about the benefits.
Research has shown that bed-sharing increases the frequency and duration of breast-feeding, and that the mother's breathing and body heat help regulate the infant's breathing, heart rate and body temperature.
"Human mothers and babies are biologically designed to sleep together," says James McKenna, a professor of anthropology and director of the Mother-Baby Sleep Laboratory at the University of Notre Dame. "It's the species-wide, normal human pattern. It's only been in the last 100 years or so that any culture has departed from that."
Indeed, bed-sharing has remained popular among some U.S. demographic groups all along.
In another study that appears in the same issue of the journal, researchers interviewed 369 women living in Washington, D.C., and found 48 percent routinely shared a bed with their infant.
The women were mainly black (82 percent) and poor (68 percent reported incomes below the federal poverty level). About 16 percent of the women were Hispanic and 2 percent were white, Asian or another ethnicity.
Black women, regardless of income, were the most likely to sleep with their children, the study found.
The interviews, which were conducted in 1995 and 1996, asked mothers about their sleeping arrangements at three points: just after delivery; between three and seven months after their baby's birth; and between seven months and one year.
Of those who shared a bed with their infant at three to seven months, 75 percent continued to do so at seven months to one year.
"Because it is so common, this is something we really need to learn more about," says Ruth Brenner, lead author of the study and a pediatrician and epidemiologist at the National Institute of Child Health and Human Development.
In the first study, Willinger and her colleagues based their findings on a telephone survey of 8,453 parents and other nighttime caregivers in the United States. The annual survey was first conducted in 1992.
This survey also found that black women were the most likely to share their bed with a baby. About 31.3 percent of black mothers shared a bed with their infant in 2000, up from 21.6 percent in 1993-94, the first year the data were available for use in comparison.
While it sounds like a large increase, the uptick is not statistically significant, Willinger says. Nor was a slight dip for Hispanic women statistically significant: About 15.2 percent of Hispanic women practiced bed-sharing in 1993-94, while only 13.2 percent did in 2000.
However, Willinger and her colleagues did find meaningful increases in bed-sharing among white mothers and Asians. About 32.1 percent of Asian mothers reported they usually shared a bed with their infant, up from 11.5 percent.
And 9.6 percent of whites shared a bed with an infant, up from 4 percent in 1993-94.
The survey also found that mothers less than 18 years old were 2.5 times more likely to have their infant sleep with them. And parents in households making less than $20,000 a year were 1.5 times more likely to share beds with their children.
Researchers aren't sure the reason for the increase, although it coincides with an increase in breast-feeding and reports that sharing a bed with baby facilitates nursing. The media, notably the "baby press," have also helped promote the views of bed-sharing advocates, Willinger says.
The American Academy of Pediatrics warns that bed-sharing "may be hazardous under certain conditions," and recommends that parents may want to consider alternatives, such has placing a crib next to the parents' bed.
But McKenna, who has published more than 20 papers on the issue during more than 30 years of research, says Western culture has defined the issue in terms of the dangers while neglecting thousands of years of evolution and experience.
Yes, babies could get trapped by furniture near the bed, so parents should make sure this isn't a possibility. And yes, parents who are drunk or taking drugs could roll over and smother a child, so they should abstain if they share a bed with an infant.
"Just like we've learned there are safe and unsafe ways of putting babies in cribs, we have learned there are safe and unsafe ways of sleeping with babies in bed," McKenna says. "A mother sleeping with her baby is absolutely, inherently protective."