Kevin and Katie Amos waited two years to start a family so they could save for their three-bedroom colonial in Willowick, Ohio, and they deliberately put only two years between Nicholas, 3, and Ryan, 1. "I think that as they grow," says Katie, 33, "they'll have a lot in common." For most families, the picture would be complete--the average American mother has two kids--but the Amoses recently decided to try again before Katie turns 35. It's not because they want a daughter, although Katie's aching to buy "those cute little girl things." They just want one more bundle of joy. That moment in the delivery room, says Kevin, "really is Christmas morning."

The Amoses sum up the way most Americans make decisions about how many kids to have: some planning, a lot of longing and a little luck. Wanting a child of a certain gender is just one of many factors that influence couples. More important these days, experts say, is having money in the bank. According to the Agriculture Department, it can cost as much as $250,000 for a middle-class married couple to raise a child to the age of 17, and that doesn't include college (now about $40,000 per year with room and board at private institutions). The mother's age and career also play a critical role; many women put off marriage and child-bearing until they reach certain career goals, and that can mean first-time motherhood in the biologically risky mid- to late 30s. Stepfamilies--common in the age of divorce--face their own set of questions. Are his and hers enough? Do we need our own to be a real family? And medical advances also raise troubling issues. More children born with serious birth defects are surviving longer. Their parents must decide whether to devote all their resources to the needy child or try for a healthy child to "balance" the family.

For most of human history, these questions were inconceivable. Family planning boiled down to one rule: the more the better. "You had to have more kids because of high childhood-mortality rates," says Stephanie Coontz, a historian of the family at Evergreen State College in Washington. In an agricultural economy, parents needed lots of kids to work the fields. That changed with the Industrial Revolution and the rise of the middle class in the 1880s, Coontz says. "Kids became more of an expense and less of an asset," she says. Family planning was controversial in the first half of the 20th century, and the postwar baby boom created a dramatic spike in births that slowed after the arrival of the birth-control pill in 1960. In the past 30 years, as more well-educated women entered the work force, the birthrate crashed from about three children per mother to about two--what demographers call "replacement level" for a couple.

One of the ironies of modern family planning is that the richest parents, who can afford big families, generally have the fewest children. "Conspicuous consumption tends to show itself in the schools these people's children attend, rather than the sheer number of children," says Kristin Moore, the president and senior scholar at Child Trends, a nonprofit research center in Washington, D.C. These parents tend to have higher expectations of what they need to give their kids a good life: summer camps, private schools, tennis and music lessons.

Wealthier, better-educated parents are also likely to start later, which automatically limits the number of children they can have. After Susan Cook, 47, a lawyer turned full-time mom, gave birth to her son, Dylan, now 6, she tried for a second child. When infertility treatments didn't work, Cook and her husband, Drew Fine, 41, a lawyer, decided to adopt a girl from China. Their daughter, Marissa, will be 3 in March. Cook and Fine didn't want to stop at one because they both came from families with three children. There are no more kids on their radar screen, however. "I sort of feel like I've pushed it with adopting at 45," Cook says. Even with only two, she feels blessed. "I wanted to have kids from the time I was old enough to understand the concept of having kids," Cook says. "College, law school--it was all sort of a prelude to being a mom."

Many parents worry that stopping at one dooms their kid to all kinds of lifelong emotional problems despite extensive research showing that only children are no more likely to be troubled than other kids. But as the only-child family becomes more common, parents are beginning to see the pluses. Karen Eubank, 48, a freelance stylist in Dallas, and her husband, Richard Barcham, 49, a sales agent for a speakers' bureau, kept waiting for the "right" time to start their family. But when she hit 40, she knew the waiting was over. Luckily, she soon became pregnant and gave birth to their son, Rowan, now 6. Almost immediately, she thought about having a second child because, as an only child herself, she wanted Rowan to have a sibling. Then she read about the high rate of birth defects to children born to older mothers. Money was also a factor. "In our situation," she says, "we couldn't have sent two children to private school and afford to do all the things you want to do with them in the summer."

These days, parents who want to break the small-family mold have to plan early. A generation ago, families with six kids were not uncommon. Today a big family has three children. But that hasn't stopped Lonny Green, 42, and his wife, Carolyn Dean, 38, from filling their Richmond, Va., home with six kids--all under 8. Dean, a nurse, was one of nine kids and loved it. Green, a doctor who grew up with two siblings, says that watching the fun his wife's family had convinced him bigger is better. They're even thinking about having two more. "I honestly think the biggest change is zero to one," Green says. "From then on, it's just put one more in the pile. It's just a little more chaos." And they've already got the wheels they'll need: an old van they bought at an auction two years ago. It seats 14.