(HealthDay)—Despite what's often portrayed in movies and on TV, most women can't orgasm with penetration alone during sexual intercourse.
And simple anatomy is to blame, a new evidence review suggests.
Each woman's ability to orgasm during sex depends almost wholly on physical development that occurred while she was still in the womb, according to the review authors.
During gestation, the clitoris begins to drift up and away from the vaginal opening, the researchers said.
But among women whose clitoris drifted too far up, it may be very difficult or even impossible to have an orgasm during sex, because traditional lovemaking doesn't provide enough friction to stimulate the clitoris, said Dr. Maureen Whelihan. She's an obstetrician and gynecologist in West Palm Beach, Fla., and an expert with the American College of Obstetricians and Gynecologists.
"It's not her fault. She was born that way," said Whelihan, who was not involved with the research but reviewed the findings.
The researchers said they have figured out the distance between a woman's clitoris and her urinary opening that can predict whether she will be able to orgasm during sex, without any additional stimulation.
The "magic number" is 2.5 centimeters—slightly less than 1 inch, said Elisabeth Lloyd, who was not involved with the new study. Lloyd is an affiliated faculty scholar with the Kinsey Institute for Research in Sex, Gender and Reproduction at Indiana University-Bloomington.
"It's so strong a correlation that if you give us a woman who has a distance of 3 centimeters, we can very reliably predict she won't have orgasm with intercourse," Lloyd said. "Women can do this measurement themselves or with their partner, to help explain their own sexual experience."
Other factors, such as penis size, the skill of the sexual partner or the intensity of desire "might have some effect, but it really is the anatomical distance that seems to be predictive," Lloyd said.
Exposure to male hormones in the womb increases the amount of drift, Lloyd said. "If she's exposed to a lot of androgen, the clitoral bud migrates far away," she said.
Between 70 percent to 90 percent of women are unable to achieve orgasm with penetration alone, Whelihan said.
"Of those that claim they can have purely vaginal orgasms, 90 percent of them say they have to be on top," she added. "Guess what? When you're on top, sitting on the partner's erection and grinding on his abdomen, it's really not just a vaginal orgasm. You're rubbing your clitoris on his abdomen or pelvis."
Nine out of 10 women in her practice have had an orgasm during their life, Whelihan said, but nearly all needed direct clitoral stimulation to achieve it.
What about the G-spot, the erogenous area purported to exist inside the vagina? Autopsies haven't consistently supported the existence of the G-spot, the evidence review said.
A majority of sex experts don't believe there is such a thing, Whelihan said. "According to most of the experts, we believe if the G-spot exists then it only exists in a few women," she said.
Couples determined to achieve female orgasm during intercourse should start paying more attention to the clitoris, Lloyd and Whelihan said.
Couples can use positions where the female is on top, which allows the woman to get more friction against her clitoris. Or they can use a sexual position that allows either the man or the woman to rub the clitoris during sex, either with fingers or a sex toy, Whelihan said.
"There are many ways to have an orgasm where she's having hers while he's having his," she said. "Couples should not focus on something that will never change anatomically, and instead find ways to allow for some type of clitoral stimulation during penetration."
However, couples also should remember that orgasm with intercourse is not necessary for a woman to have a healthy or enjoyable sex life, Lloyd added.
"I think this approach is traditional, and it's very common, but it's problematic. We've learned in our research there are so many women who do not have orgasm with intercourse on a regular basis," Lloyd said. "To put this banner of healthiness as having orgasm with intercourse kind of stacks the deck against these women who, because of their anatomy, cannot have orgasm with intercourse."
The evidence review was conducted by Leslie Hoffman of the department of anatomy at Indiana University School of Medicine, and colleagues. The report was published online April 4 in the journal Clinical Anatomy.
Explore further: There's no such thing as a vaginal orgasm, review finds
For more on female orgasms, visit the Society of Obstetricians and Gynecologists of Canada.