"You've brought this new life into the world, and you think, 'How can anyone take care of this baby as well as me?' So to put her in day care is, to some extent, a leap of faith," says Nikki Horberg Decter, a lawyer in Boston.

Decter, 33, took that leap a year ago when she and her husband, Jonathan, chose a child care center for their daughter.

REASSURING RESEARCH

About two-thirds of U.S. moms with preschool children are employed, and many still worry that not having a full-time parent at home will hurt their children. "I worry that we could do it better, that my husband and I are missing out on time with her we will not get back," Decter says.

Worry has been an ongoing theme since baby-boom women surged into the labor market. The percentage of mothers of preschoolers with jobs jumped from 30 percent to 60 percent from 1970 to 1990 and has edged up to 65 percent since then. But today's parents have an advantage: Research on day care has greatly increased in the past decade, and findings have been mostly reassuring.

And sometimes the children themselves offer the greatest reassurance. The Decters chose a day care center with a loving climate that provided at least one adult for every three babies. Their daughter, Lila, 15 months, has been gloriously happy there, Decter says. "She's been known to get upset when her father picks her up."

But that doesn't mean the concern is over. Babies' needs change. Now the Decters feel the center isn't intellectually stimulating enough for a toddler. "I worry that the programming is not state of the art, the enrichment activities are not up to snuff," she says. So they're shifting to another center in the fall.

"Lila has benefited from day care," Decter says. "But you want the very best for them, and you have to keep evaluating things."

UNSETTLING STUDIES

Some parents were taken aback by a new finding in March from the longest, most thorough U.S. study on day care. The federally financed study, which tracked 1,364 children from birth through elementary school so far, has gathered a trove of information about their parents, day care arrangements and how the children fare emotionally and academically.

High-quality preschool care of any type correlated with slightly better vocabulary by the fifth grade.

What raised eyebrows was that the more time preschoolers spent in day care centers — not with nannies or at family day care homes — the more aggressive and disobedient they were in sixth grade, according to teacher reports. But the difference was not statistically significant unless preschoolers were in group care for four years, says James Griffin of the National Institute of Child Health and Human Development.

Also, the "behavior problem" score differences were small. On a scale of zero to 100, the average score is 50. Those in group care for four years averaged 52 — still in the normal range, Griffin says. "These are not the bullies teachers dread," says study co-author Margaret Burchinal of the University of North Carolina-Chapel Hill. Kids who score 60 are referred to counseling for possible behavior disorders.

Still, the finding is worrisome, says senior author Jay Belsky of Birkbeck, University of London. He likens a little more disruption in many classes to a little more pollution from many cars — it can add up.

Kids in long-term group care may become more peer-oriented, says Burchinal. If there's not enough individual attention, misbehavior can be contagious.

But a key reassuring conclusion of the study is that quality of mothering — how sensitive a child's mom is to his needs — far outweighs anything about day care.

Another large study out in February tracked kids only to kindergarten. It, too, found long hours in group care correlated with more behavior problems, but not for blacks or English-speaking Hispanics, who had academic benefits.

THE FIT COUNTS

Any type of child care can be negative — or positive — for certain children, says Karen Heying of Zero to Three, a national nonprofit group. A good day care center can offer mental stimulation, she says.

The fit counts, she adds. "A very social child who blossoms in groups might do well in a center. ... A more low-key kid who doesn't like a lot of change or stimulation would probably do better in a smaller setting."

And soaring costs, along with limited availability, can restrict parents, says Gina Adams, a day care specialist at the Urban Institute.

Research is rapidly growing on how day care affects children, with studies under way that are more scientifically sound than ever, says Martha Zaslow, vice president for research at Child Trends, a group that tracks children's well-being.

Still, decisions come down to individual families, and for many parents, they're not easy ones. "It's hard to look at your baby lying in a bassinet," says Decter, "and try to figure out what she's going to be like and what will be best for her."

WHERE THE KIDS GO

Percentage of preschool children with employed mothers in different kinds of day care:

Relatives: 24.8 percent

Child care center or nursery school: 24.3 percent

Father: 17.5 percent

Nanny or family day care home: 17.2 percent

Other (such as no regular arrangement): 13 percent

Mother while working: 3.2 percent

Source: U.S. Census Bureau

SEEKING A GOOD FIT

Use these profiles, provided by Zero to Three, to help decide what type of care might best suit your child. Some questions to answer:

1. WHAT IS YOUR CHILD'S PERSONAL STYLE?

Adaptable; flexible, good with transitions

Look for large group setting providing social interaction and new experiences.

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Cautious; thoughtful, has difficulty with transitions

Look for small group settings that are less overwhelming.

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Feisty; passionate, has difficulty controlling feelings, wants and needs

Look for programs where each child has a primary caregiver who knows and understands them.

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2. HOW DOES YOUR CHILD REACT TO THE WORLD?

Big reactors communicate their feelings and don't hold back.

Look for caregivers who respect your child's emotions but help him express them in a more controlled way.

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Low reactors fuss very little, play quietly and sleep a lot.

Look for caregivers who gently introduce new activities.

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3. HOW ACTIVE IS YOUR CHILD?

High activity children are always on the go.

Look for individual schedules, safe exploration and physical play.

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Low activity children sit quietly, exploring through their eyes and ears.

Look for caregivers who engage children in sensory-rich activities.

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4. HOW SOCIAL IS YOUR CHILD?

Super-social children enjoy approaching new people, are happiest interacting with others.

Look for opportunities for group play.

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Take-it-slow children are shy around strangers, need time to warm up.

Look for programs that allow children to choose when they'd like to play with others.

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5. WHAT'S YOUR CHILD'S TOLERANCE FOR CHANGE AND CHALLENGES?

High tolerance children aren't bothered by change, are adaptable, cope well with frustration.

Look for mixed-age settings that offer enjoyable challenges and stimulation.

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Low tolerance children are sensitive to change, get upset when daily routine is altered, become easily frustrated and impatient.

Look for same-age settings with developmentally appropriate toys and activities.

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6. WHAT ARE YOUR CHILD'S UNIQUE NEEDS?

Sleep: Look for settings conducive to naps.

Cultural identity: Look for caregivers who speak your language, are open to aspects of your culture, have values consistent with your family's.

Special needs: Look for an accessible setting, caregivers who understand the medical procedures and technology your child needs and who are willing to help your child develop skills that match what your family believes are important.