Iris, a young woman from North Carolina, can't understand why she has no sexual desire -- she is only 30 and in love.
"It distresses me because I will marry soon and I know this will create a lot of tension in my marriage," Iris (not her real name) wrote to ABCNews.com.
Iris is one of an increasing number of women in the prime of life and at the height of their fertility who have lost their sexual desire, according to medical experts and a growing body of research.
Stress, depression and bad relationships can contribute to low libido, but often birth control pills or antidepressants are the culprit.
A 2010 study by the International Society for the Study of Women's Sexual Health found many young women experienced guilt and distress over their sex lives.
A cross-sectional study of 31,000 U.S. females 18 and older published in 2008 in Obstetrics and Gynecology magazine, about 43 percent of women reported sexual problems.
Of those aged 18 to 44, about 10 percent complained about low sexual desire -- or what is called hypoactive sexual desire disorder (HSDD), according to the study, Prevalence of Female Sexual Problems Associated with Distress and Determinants of Treatment Seeking (PRESIDE).
"It's a real diagnosis," said Dr. Carolyn Nemec, a women's health specialist in the department of family medicine at the Cleveland Clinic. "It's very surprising and we are missing the boat if we don't talk about younger women."
"We always think of it as something that women go through at menopause, but millions of American women are affected," she said. "It's a complex issue and women's libidos are complex."
In 2002, the American Psychiatric Association categorized HSDD in its Diagnostic and Statistical Manual of Mental Disorders (DSM) as any persistent lack of sexual desire that causes a woman distress.
The disorder was most common in women aged 45 to 64 at 14.8 percent.
It's not so much the lack of libido that characterizes the disorder, but the amount of anxiety it causes. Even though post menopausal women report less sexual desire, only about 8 percent are diagnosed with HSDD, because they are not distressed, according to the PRESIDE study.
"I think a lot of women are distressed out there and feel bad and think something is wrong with them," said Nemec. "The younger patients hear so much about menopause and a lot of these women -- aged 18 to 30 -- don't feel they have a voice. We need to focus on them."
Up to 40 percent of all women who have been diagnosed with HSDD also report depression, she said. And for those who are on antidepressants like SSRIs, as many as half can experience a decrease in desire.
Birth Control, Antidepressants Can Cause Low Desire
Birth control pills that contain estrogen and progesterone can also be to blame.
"They increase sex-binding globulin, a protein in the blood stream that binds with our testosterone and testosterone is one of the central hormones in desire," said Nemec.
When she rules out all other causes of low libido, Nemec will often recommend trying a different antidepressant or decreased dosages. Sometimes, she'll ask the woman to go on a "drug holiday," to restart her libido.
For women, sexual desire is a complex psychological and physiological phenomenon.
Some of Nemec's patients are young women who have just had a baby and trying to reignite desire after a pregnancy. But others are single women in their 20s who began having sex just to keep up with their peers.
"I call it the 'going along syndrome,'" she said. "By 17 they are sexually active and go along with sex, even if it doesn't feel good or they don't have a libido. If they go along at 18 or 20, they continue after they are married."
The danger is that if they don't deal with the issues of desire until they are older, "the husband finally gets Viagra at 60 and she's really in trouble."
That may have been the case with Joan (not her real name), a Texas woman who wrote to ABCNews.com.
"I have never had a libido, nor an orgasm," said the 52-year-old. "I have looked for ways with creams and pills but to no avail."
"I would love to find something to give me that feeling of wanting to have sex," she said.
Sometimes loss of desire is exacerbated by other physical problems. One of Nemec's patients, a 25-year-old newlywed, has a rare condition known as vulvar vestibulitis.
Jane, not her real name, has chronic vaginal pain -- a feeling of "burning and raw skin" -- and hasn't been intimate with her husband in months.
"I went off birth control pills, tried dilators and physical therapy and every cream in the world," she said. "I even tried Botox, but it didn't help."
"Every now and then I have the desire or urge, but then I just think about it being painful," said Jane, an occupational therapist.
She's lucky to have a supportive boyfriend, but it still takes its toll.
"In media everything is sex-based, and it's hard because you don't feel sexy," she said. "I have had this for so long, I am trying to build my self esteem in other ways."
For most women, low libido can be diagnosed with a five-question diagnostic tool, a questionnaire developed by the German-based pharmaceutical company Boehringer Ingelheim.
Some critics point to the pharmaceutical companies for creating new diagnoses like HSDD for the sake of drug sales.
Just last November, Boehringer Ingelheim's new drug flibanserin was hailed as the new female Viagra. A random, double-blind international study showed the drug appeared to increase sexual desire by enhancing mood.
The new drug works on neurotransmittors, not on blood flow, as does Viagra.
The PRESIDE study was also funded by Boehringer.
Study author Dr. Jan Shifren told ABCNews.com last year that women who have no sexual desire should not be "medicalized," but for those who were bothered by their lack of desire, "this group of women deserves effective treatment."
"If an effective and safe medication is identified, it should be available to women, but given potential risks and side effects, drug therapy always should be considered a last resort," said Shifren, who is director of the Menopause Program in the Massachusetts General Hospital Department of Obstetrics and Gynecology.
"Women in good relationships who are physically and psychologically healthy are generally satisfied with their sex lives -- and you'll never find this in a medicine cabinet," she said.
Too Much Pressure on Women to Be Sexy
But Judith Steinhart, a health and sexuality consultant certified by the American Association of Sexuality Educators, Counselors and Therapists, said today's young women -- and men -- feel the pressure to be "sex pots."
Steinhart argues that the medical and pharmaceutical community is dictating new norms: "Whose definition of sexual desire are we talking about?" asked the New York City sex educator.
"Is it lack of sexual desire or do we have expectations that we are supposed to be hypersexualized women at 16, 26, 36 and so on?" asked Steinhart.
"Take a look at any MTV video or at Beyonce," she said. "Who can even move that way? She works so hard on the dance floor, I'll bet she goes to sleep early, too."
Men have a built-in "biofeedback mechanism" for desire, according to Steinhart. "When they are aroused they can see their erection and it reinforces their arousal. Women don't have the parallel opportunity."
"[Young women] are so tired from working so hard that they think sex is supposed to be a release and a relief and often it isn't," she said.
"A lot of women say the best part of traveling is the hotel room with the giant bed and the clean sheets and no children crying and a husband who wants something from them," said Steinhart. "Truthfully, women have always been this way."
"One reason is stress," said the Los Angeles therapist. "It's a libido killer."
"Right now, people are losing jobs and working longer hours," said Berman. "They are doing two or three people's jobs and taking pay decreases. They are exhausted."
Women are often the primary breadwinner or are under pressure to make up for the family's financial shortages.
"From what I am seeing young women are feeling older than their age," said Berman. "They have diminished sex libido and are too tired to care."
Just because a woman has low sex drive now, doesn't mean it's permanent.
"Most people don't realize that their sex lives ebb and flow throughout a lifetime," said Berman. "It can be low this week and the next month the couple is having sex five times a week. All is not lost."
"Couples who just gave birth are unlikely to be going at it," she said. "But when the kids have left for college and there's an empty nest, there is a revival of the sex drive."