"Pay attention to the age of the relationship rather than the age of the people," said Dr. Sheryl Kingsberg, chief of the division of behavioral medicine in the department of obstetrics and gynecology at Case Western Reserve University School of Medicine. "How long has the relationship gone on?"
In the earliest stages, every relationship has the "unfair advantage" of a high level of passion, according to Kingsberg. Experts even have a word for it – limerence. But that period of intense infatuation eventually ends, usually after anywhere from six months to two years.
"The couple, at that point, is at a crossroads," said Kingsberg. "Either they move into more of a partnership and build a life together or they break up and move on."
Half of all American marriages will end in divorce, but for the millions of couples who stick with it, life can be full of challenges.
Women face numerous hurdles, from making time for intimacy amid a busy family life to maintaining good communication skills. Some worry about the heartbreak of infidelity and other women complain that they are no longer attracted to a mate because he has put on extra pounds.
"Successful couples, those who really are the happiest, know that they have to work at it," she said. "Couples who expect it will happen automatically and take each other for granted, at high risk for failing."
Ann Rosen Spector, a clinical psychologist from Philadelphia who specializes in couples therapy, said, "When the relationship isn't doing well, you don't feel good about yourself."
"Relationships are always a work in progress so when we start to take it for granted, and don't put in as much attention and care, it's not going to thrive," she said. "One of the biggest hurdles is complacency."
KIngsberg and Spector both say women are more stressed today than ever before, juggling the demands of careers and motherhood. It's easy to lose focus and put an important relationship on the back burner.
"Make time for each other and talk about issues, small, medium and large," said Spector. "Carve out time, a small amount each day. Try to build the 'we' and not always look at the 'me.'"
Take time to listen to the partner and choose the argumentative battles. "I tell couples if they are fighting about this now, do they want to be fighting about it in 30 years?" she said. "People say, 'no.' They need to take the long view. This is an investment."
"The issues are exactly the same," she said. "In the first throes of romance there is a lot of excitement and interest and I don't care if your partner is male or female," said Cantor. "When the initial excitement wears off, how do you keep the interest alive? A lot of women still think it's like a fairy tale, that they will be married and live happily ever after. A good relationship requires ongoing attention and work."
Call grandma or hire a babysitter and plan a date night. Also, make clear to children not to interrupt parent-to-parent conversations. "Kids need to learn to wait and not get their needs met instantly," said Cantor.
As for communication, use empathy and patience.
"A lot of women don't know how to get the conversation started without being perceived as nagging," she said. "Try not to open with a line that's accusatory, she said – an 'I-message rather than a you-message.'"
Still, the pressures of everyday life can wreak havoc in the bedroom.
"Stress is a libido killer," said California psychotherapist Jenn Berman.
"Right now, people are losing jobs and working longer hours," said Berman, whose radio program, "The Love and Sex Show With Dr. Jenn," airs on Sirius XM. "They are doing two or three people's jobs and taking pay decreases. They are exhausted."
Busy career women can allow cell phones and e-mail messages to interfere with their relationships.
And when the spark has fizzled, that, too, can weigh heavily on a woman.
"When there are problems in the bedroom, women are very relationship-oriented, and when things are not going well, it adds tremendous stress to a woman's life," she said. "You can be uber-successful and have a wonderful, accomplished life, but when the relationship isn't going well, it likely affects you."
She advises women to be proactive in keeping the intimacy in a relationship, "whether it's initiating sex more often or looking for ways to spice up the bedroom or better communications techniques or getting in to counseling. The key is don't get depressed, because then you feel helpless."
"Most people spend more time having their eyes on their favorite TV show than on the family and that needs to change," she said. "That's not a model for your kids to have an intimate relationship."
"Putting on a lot of weight is hard on body and hard on libido," said Berman. "Weight gain is oftentimes a sign of depression and when a person is depressed, that also kills the libido."
And when one partner is unfaithful or turns to pornography, Berman holds both responsible. "Generally speaking, it's 50-50 and when someone turns to that, it's often a symptom of a much bigger problem in the relationship."
Then, counseling is in order.
"We know that for women, the old adage, 'sex equals love rather than love equals sex,' applies," said Case Western's Kingsberg. "I don't want to stereotype men or women -- there are a lot of women who take the sexual lead and the other way around. But there are ways to communicate that."
"In first few months to a year the sex drive is high for everybody and after that there is one person who has less desire," she said. "You can make it feel less like a chore -- more than the body might want, but once it's started, its enjoyable. And the person with the higher sex drive can pay attention to the fact that quality does outweigh quantity and don't take in personally."
Communication is the key.
"Romance doesn't have to lead to a sexual encounter every time," said Kingsberg. "You can be playful, even in the busiest times of the day when you don't expect to end up in bed with each other. Send a flirtation (by e-mail) in the middle of the day, saying, 'I'm thinking about you.' "