2 to 3 years apart

This is the most common interval in American families, according to demographic surveys, and some experts feel it's the best option. "Two and a half years is great timing," says Hart, whose children are almost two years apart. "They're close enough in age to be wonderful companions  -- they'll often go to the same schools and see the same movies  -- but they're far enough apart to give Mom more time between pregnancies." And though they can fight like cats and dogs, she says, these squabbles help teach them valuable problem-solving skills.

White doesn't dismiss the sibling rivalry of closely aged kids as easily. As director of the Harvard Preschool Project, he spent 13 years observing children interact with their younger siblings. What he found was more aggression among kids close in age, as well as increased chronic stress among the caregivers. He saw the dynamic in action in his own home  -- he had four children within six years. "Never in my wildest, craziest dreams would I have inflicted that on a mother if I knew then what I know now," he says.

3 or more years apart
A larger age gap can alleviate some of the competition that comes with close-in-age kids, says White. "A four- or five-year-old will treat a one-year-old as her own, showing genuine affection, empathy, and compassion." An age difference of five or six years, he says, is even more stress-free.

Since the older sibling has had ample time to bond with Mom and Dad, she may now be happy to take on the role of mentor. "The age difference turned out to be terrific for us," says Marjorie Ingall of New York City, whose kids are three years apart. "Josie can help  -- fetching diapers or grabbing a baby spoon. She was more excited than sulky about having a sister."

Betsy Murphy of Coral Gables, Florida, waited four and a half years between her sons Teddy and Anderson, partly because she'd been stung by having Teddy only 21 months after big sister Caroline. "I've only seen my husband cry three times," she says. "One of those was when we brought Teddy home from the hospital and he saw the look on Caroline's face when I was holding Teddy in my bed."

But it can be complicated to meet the needs of kids who are at different developmental stages  -- one is creating a Lego castle while the other is trying to eat it  -- and they may not share friends or hobbies. Some moms also say it's hard to get back into baby mode after enjoying the freedom of a more self-sufficient child.