Historically, husbands and wives have bred big for a variety of reasons: lack of birth control, a need to staff up the farm, the desire for a net gain after losses to the bubonic plague, the hope for more willing hands to push their own wheelchairs later. But today, expanding the family seems to be less about what your offspring can do for you and more about what you can do for your offspring. More kids doesn't mean more help slopping the pigs—it means more playdates, more soccer games, more tutors, more iPods, and more 529 plans. It means tripling or quadrupling the custodial expense of having a family, financially, spatially, and psychologically. So when a couple make the decision to have another, they're not just adding an extra mouth to feed—they're biting off more to chew for a lifetime.

Divides have long existed between the haves and the have-nots, the stay-at-homes and the execu-moms, but now there are new socioeconomic rifts—the only-child parents and the twofers, the twofers and the multiples. The generation that grew up with The Brady Bunch, The Partridge Family, and Eight Is Enough is finally getting its chance to populate the prairie, and some of us are going for it. While a 2004 U.S. Census Bureau survey on fertility reported only a slight increase in family size since 1990 (51 percent of women in their childbearing years had two or more kids, up from 48 percent), the media has declared a baby boom, particularly among the affluent. And any suburban ob-gyn worth her weight in Similac will tell you three is the new two (or even four is the new three).

Today's grandparents bemoan the conventional wisdom that kept their families small in the '70s—namely, that parents of more than two were either blithely ignorant or Catholic. Now, in some circles at least, the feeling is very much the opposite. In 2005, New York Observer style reporter Simon Doonan wrote, "The critical third child—quite possibly the status symbol of this decade—will get you more Park Avenue cred than a fleet of Bentleys. The third child screams, 'My apartment is massive, my SUV spacious, my cash unlimited!'" Considerations of the social ladder aside, if you're contemplating a second, third, or fourth child, what exactly are you getting into? How will your life change? The answer is that most things will get worse: Chances are you'll sleep less, spend more, weigh more. The upside? Harder to tally. One mother says the biggest difference between having two and three children is that there is "more love in the house" with the latter. Great for the soul; not so great for your ass(ets). To determine exactly how the rest of your life might change, we gathered some research—some anecdotal, some scientific, some decidedly not, and some online through a poll of 500 Cookie readers.

Here are the results:

College savings

Savingforcollege.com can calculate how much you will need to set aside per child, based on how much time you have before your kids graduate from high school and whether you expect them to attend private or public schools. It estimates earnings of 7 percent interest on every dollar you put away now, and a 6 percent increase in college costs per year. What's the damage? To put 6-year-old Johnny through Middlebury in 12 years, you'll need to save $1,456 a month until he gets that degree. Or let's say you have four children—ages 2, 4, 6, and 8—and you'd like to send each to a private institution, on you. Brown University, to pick one, charges $46,340 a year in tuition, room, board, and student fees. Sending all four kids there would cost you a total of $1,744,261. Vocational school isn't looking so bad.

Coffee consumption

If it's true that behind every good man stands a great woman, then surely behind every great woman is a great big cup of coffee. Or so we thought. Even though a shocking 31 percent of Cookie poll respondents said they don't drink any coffee at all, 29 percent of those with more than three children down three or more cups a day, while only 9 percent of their single-child cohorts made the same claim. Some mothers have an almost comical relationship with their morning beverage. Sara Corbett, a mother of three in Portland, Maine, has a coffeemaker on her bedside table and doesn't put toe to floor before consuming her first cup. And Jennifer Mackinnon, a mother of two in Los Angeles, drinks one cup at home as she gets dressed, fills a travel mug for the trip to her older son's school, then returns home for a top-off—car running, younger son waiting—before the final drop-off.

Pounds gained

According to a 2004 Duke University Medical Center study, the more children women have, the greater the risk they will become obese. Researchers found that, on average, women faced a 7 percent increase in risk per child, which they attributed to "busier lifestyles that may include more fast food and less time for exercise." The Cookie poll, while not addressing obesity specifically, dug up parallel information on weight gain. Among respondents with more than three children, 33 percent said they have put on 20 pounds or more since the birth of their first child, while only 2 percent of those with just one child reported the same weight gain.

Sex

Finally, the good news: If you have more kids, you'll have more sex! Or wait—is it the other way around? Leave it to the modern woman to forget the connection. Thirty-six percent of mothers with only one child who responded to our poll claimed they have sex once or twice a week. Sounds reasonable, right? But 46 percent of the women with four or more children say they have sex that often. Are they just friskier? How do they have the energy? How do they find the time? Come to think of it, how did they find the time to go online and respond to our poll?

Marital satisfaction

Everyone knows having a child to "save the marriage" works about as well as garlic does on evil spirits. But while getting pregnant is no cure for an ailing relationship, could adding another bunk to the bedroom actually destroy it? According to our poll, parents of two are happiest—92 percent rated their marriages "satisfactory" or "as strong as ever," whereas only 61 percent of those with four or more kids gave their marriages the same rating. A study published in the Journal of Marriage and Family reported a negative correlation between the number of children in a family and marital satisfaction, especially among parents in "high socioeconomic groups," where "the restriction of freedom" was more pronounced. "It might have to do with the loss of a previous life," says Jean Twenge, the study's author. "Maybe they were used to taking expensive vacations or eating at restaurants a lot."

Vacations

Amusement-park family passes and booths at restaurants are mostly designed for families of four—even shopping carts hold two, max. But that doesn't deter parents who love traveling with a large family. "We bought a time-share and try to vacation there at least two weeks a year," says mother of four Nicol Shults, of Lexington, Kentucky. "It makes vacations easier and more fun." But Vanessa Flannery, also of Lexington, who has three kids under the age of 6, doesn't bother: "I look at my friends with two kids and think they have it so easy." And those with one? "Please," she says. "Put her on your back and go to Europe."