The female orgasm is already known by researchers to have a powerful physiological effect. The strong muscular contractions it produces help to push sperm from the uterus into the fallopian tubes, where eggs are fertilised. Aided by an orgasm, sperm can travel this distance in 15 minutes - which is 50 times faster than it would take if they swam without help.
Now a study from the Hammersmith Hospital, London, suggests that not coming to a climax during sex may reduce the chance of a woman conceiving. Dr Laura Quinton, research fellow at the hospital's in-vitro fertilisation unit, has found that women with unexplained infertility were more likely not to have orgasms than fertile women.
The findings indicate that many couples trying to have a baby need more information about how the woman can achieve orgasm - not only to increase her pleasure, but also to maximise her chances of conception.
"Sex is at the core of fertility," says Dr Quinton. "Yet it is often treated as a taboo subject when couples have sub-fertility problems. More discussion of women's perceptions of orgasm and how orgasms can be achieved could be helpful."
While it is widely accepted that male sexual dysfunction, particularly erectile and ejaculatory impotence, causes 5 per cent of all infertility, doctors do not generally believe that a woman's sexual functioning - how she experiences sex - is important in influencing fertility. This view could change.
The results are likely to be controversial, however, because they raise the unfashionable concept that there might be a psychosexual aspect to female infertility. In the Fifties, it was common for gynaecologists to brand infertile women as neurotic or frigid, adding to their already heavy burden.
In 1964, for example, an American gynaecologist, Irving Fischer, wrote in the Journal of Fertility and Sterility that for many women "behind the conscious desire to have a child, there may be a deeper, unconscious wish not to have one".
He went on to say that women with "psychogenic sterility" could be divided into two groups: the weak, emotionally immature, dependent, overprotected woman and the ambitious, masculine, aggressive, dominating, career-type woman.
But as many of the causes of infertility have become understood, infertile couples are no longer branded as psychologically inadequate or sexually dysfunctional. The accepted wisdom now maintains that infertility causes sexual problems rather than vice versa. Doctors also recognise that it is not necessary for a woman to have an orgasm to get pregnant, since anorgasmic women (women who have never had an orgasm) have become mothers.
Dr Quinton agrees that the reason why surveys have shown that infertile women enjoy sex less than others might well be to do with feeling that they are under strong pressure to conceive. But she now thinks that might not be the full story.
"The other theory is that there are aspects of female physiological function which are important to fertility," she says. "In men, it has been proved that they can have problems with erection and ejaculation. There might be female correlates, such as the strength of orgasm or other subtle aspects of sex that we don't recognise."
Her team compared the frequency and nature of orgasm among 219 infertile women and 84 fertile women. At first there seemed to be no difference. But when the infertile group was sub-divided into those with blocked fallopian tubes and those with unexplained infertility, strong differences became apparent.
"The women in the unexplained infertility group had significantly poorer overall scores in the questionnaire on sexual function than the fertile group or those with tubal damage," says Dr Quinton. "They were much more likely to be anorgasmicthan the fertile or tubal damage group.
"Statisticians analysed our data and said that the likelihood of our results occurring by chance was less than one in 1,000."
Dr Quinton was still surprised by the results. "We did not expect there to be any difference when we began the work. We thought it would dispel the myth that orgasm was important."
She and her research team are now beginning a prospective study involving 40 women, recruited through a magazine article, who have no known fertility problems and are trying to become pregnant. The team will investigate whether those with "normal" sexual function are quicker in conceiving than those who are considered "dysfunctional".
But what is "normal" sexual function? The female orgasm has been subjected to considerable study since the war. Masters and Johnson, the pioneers of sex research, dispelled the notion of two types of orgasm, originated by Freud, who defined them as immature (clitoral) and mature (vaginal, achieved through intercourse).
The Freudian myth of the vaginal orgasm, however, is still alive and well. The latest research from the Kinsey Institute in Bloomington, Indiana, shows that 10 per cent of women have never had an orgasm by any means. It found that many couples still feel the woman ought to be able to climax through "penile thrusting" alone. Yet between 50 and 75 per cent of women who reach orgasm require stimulation of the clitoris, by the woman or her partner, either before, during or after intercourse. "All these behaviours are 'normal' and do not mean that either the woman or her partner is deficient in any way," says the Kinsey Institute's New Report on Sex, published in 1990.
Doctors at the Hammersmith's IVF unit are also trying to discover whether there is any truth in the commonly held view that stress contributes to infertility.
"At our infertility clinic couples are always looking for reasons why they are having difficulties in becoming parents. They say to us, 'Is it our fault? Are we trying too hard? Are we too anxious?' " says Dr Enda McVeigh, research fellow in obstetrics and gynaecology. "We tell them no, we do not think it is has anything to do with that.
"Now we are trying to obtain proof. We are just finishing a two-year study of 90 patients. We have been measuring the levels of endorphins, prolactin and cortisol in the blood and ovarian fluid, three hormones that rise with stress. We already know that if they go very high, they stop egg production. We want to know whether, if they are moderately raised, that will affect the quality of eggs."
Patients' hormone levels and psychological state are measured at the beginning of a three-month treatment cycle and during it. Then the team looks to see whether those with higher stress levels have a lower rate of pregnancy than those without high stress levels. The early results have shown no correlation.
Dr McVeigh says: "The relationship between stress and fertility is obviously a complex one, since many couples in stressful jobs do manage to become parents and rape victims have a higher pregnancy rate than women of the same age who are trying to conceive. We would like to go a little further towards understanding it."