I remember expecting that after I had kids I might be too busy or too tired or too saggy for a robust sex life, but I didn't think in my wildest dreams that I might be too bitchy. Research shows that two-thirds of married folks feel their marriages suffer significantly in the aftermath of the birth of babies, and this includes experiencing less frequent and less satisfying sex.

Anyone who has ever had a watermelon-sized miracle come out of her body knows that childbirth, and the stress of child-rearing that follows, can test even the most vigorous of sex lives. But getting or keeping our sex life on track has—you guessed it—important implications for children's happiness. Read on to learn how to put a little va-voom back in your bedroom.

Research does show that the quality of a couple's sex life tends to get steadily worse over the course of a marriage, so if it isn't as hot as it used to be, you aren't alone. Sex is both less satisfying and less frequent when you're married with children. About 50 percent of parents in one study described their sex life as "poor" or "not very good" when their first baby was 8 months old.

How often we do it tends to decline the longer we stay married. Most couples have a lot of sex in the first year they're married (the "honeymoon effect") but all that rabbitlike activity drops off precipitously around the end of the first year. Once we have children, biology starts to grind on us.

Rutgers University anthropologist Helen Fisher, who studies the brain circuitry of romantic love, says millions of years of evolutionary adaptation account for a couple's divergent sexual interests after kids are born. For instance, when a woman is nursing and holding her child, levels of the hormone oxytocin surge, leading to intense feelings of attachment. Testosterone levels, which are related to sex drive, plummet.

"Mom's not just overly tired and making excuses—she's drugged," Dr. Fisher says in an interview in The Wall Street Journal. "From a Darwinian, evolutionary perspective, if Mom's not there to take care of the baby, it will get eaten by a lion ... both parents are fighting a basic evolutionary mechanism that evolved to strengthen the mother/infant and parental bond, not the sexual bond."

One important predictor of how often a couple does the no-pants dance is how happy they are with the marriage. Increasing the level of happiness can, to a certain extent at least, put our paltry sex lives back into our control, more so than the instinct to save our babies from lions. There are some other important gender differences at work, too, which often leave their mark in the bedroom. Men, on average, want to have sex four times a week, while women would be happy with—I hate to break it to you boys—just once. Attention new parents: If you want a more boisterous sex life, here are some insights that might help you stimulate your significant other's libido.

  1. Women often resent men who don't help out more around the house or with the children, and that anger usually does not fire a parallel passion in the loins. If you are one of these men and you want more sex from the mother of your children, rethink your ideas about foreplay:
  • Pre-kid romance: bringing her flowers, commenting on her hot tush, nuzzling her neck.
  • Post-kid romance: folding the laundry without her asking and putting it away, then noticing that she seems exhausted and running her a bath while you take on whatever other tedious household task needs to be finished. She'll be panting in the bedroom by the time you're done, I promise.
  1. Another gross generalization supported by scientific research: Sex tends to mean different things for men than it does for women. Women see sex as an expression of pre-existing emotional intimacy, while men see sex as the path to that intimacy. Women tend to be more satisfied with both sex and marriage when their partners behave lovingly and affectionately toward them, and commonly cite things that indicate verbal intimacy—like having a heartfelt conversation—as something that leads to sexual activity. Verbal intimacy—those deep discussions where you really connect in a positive way—really stokes the fire. If you aren't having sex as often as you'd like, start by connecting with your partner through conversation.

Despite these dismal gender differences, here's some better news: Sexual famine is not necessarily a given after we have kids. The majority of couples get back to having sex about twice a week. But until biology starts working for you again, find a way to resist those forces that throw water on the flames—social, interpersonal and biological—the best you can.

If you can't seem to make it happen any other way, schedule time for sex with your partner. It may seem cold and unromantic, but the fact is, any kind of sexual interaction triggers a wash of feel-good chemicals in our brains. There is evidence to suggest that once you are happily married, you'll do best to think of sex like exercise—we might be loath to get off of the couch at first, but once we get going, we'll be glad we did. (The first push-up is always the hardest.)

Because intimacy in your relationship is so strongly related to marital satisfaction, it is important to keep those hearth fires alive if we want our marriages to survive until our children leave the nest (when many marriages experience a renaissance).

May your relationship's renaissance happen this week!

Republished with permission from Christine Carter and the Greater Good Science Center.

Published on Feb. 1, 2011; updated on May 12, 2014.

Dr. Christine Carter

Dr. Christine Carter, a sociologist at UC Berkeley's Greater Good Science Center, is the author of "Raising Happiness: 10 Simple Steps for More Joyful Kids and Happier Parents." She also writes a blog for Greater Good, which is syndicated on the Huffington Post and PsychologyToday.com. Dr. Carter has been quoted in Women's Health and Parenting magazines, The New York Times, Los Angeles Times, USA Today and dozens of other publications. She has appeared on the "Oprah Winfrey Show," the "Rachael Ray Morning Show," "The Daily Show with Jon Stewart," "CBS Sunday Morning," "ABC World News with Diane Sawyer" and NPR. Dr. Carter teaches parenting classes online throughout the year to a global audience on her website.