The best way to get pregnant is to have a simultaneous orgasm with your partner, scientists have discovered.

The findings suggest that couples who try for a child by having sex to order at the 'right time of the month' may struggle to conceive.

Doctors investigating a theory that earth-moving simultaneous orgasms assist fertility found that women who experienced pleasurable sex retained more active sperm from their partners than those that did not.

Dr Jacky Boivin, from the School of Psychology at Cardiff University, studied 71 women with an average age of 30.

The women were asked to go to a clinic within two or three hours of having sexual intercourse for a post-coital test to measure how much sperm had survived in the cervical mucus.

They were also asked about whether their sexual encounter had been satisfying, and whether and when they had achieved orgasm.

Dr Boivin found that almost half the women who experienced disappointing sex retained no sperm at all, compared with only one in ten women who had enjoyed a satisfying sexual encounter.

Pregnancy is more likely to follow good sex

Describing her findings at the British Association Festival of Science at Cardiff University, Dr Boivin said: "The study results showed a significant and direct relationship between the quality of the woman's sexual response and the number of sperm found in the cervical mucus."

There are two main theories to explain the link between pleasurable sex and sperm retention.

Suck in hypothesis

The "in-suck" hypothesis argues that contraction during orgasm draws in the sperm after ejaculation.

Arousal has also been shown to reduce acidity inside the vagina. According to this theory an acidic vagina lessens the chances of sperm surviving to the reproductive tract.

However neither of these theories has been proven beyond doubt.

Dr Boivin added: "If the number of sperm in the cervical mucus is an important predictor of future fertility, then the findings outlined may have important implications for infertile couples.

"In such couples lovemaking frequently occurs under stressful conditions as it is determined by the fertile period rather than sexual desire.

"The combined effects of borderline fertility with such stress may be sufficient to impoverish the couple's sexual response and effect a reduction in. fertility."

Relaxation is crucial

Dr Martyn Walling, a Lincolnshire GP and member of the primary care gynaecology group, said: "The more relaxed people are when they are trying for pregnancy the more likely they are to get pregnant.

"If they are having a simultaneous orgasm they must be really relaxed."

Dr Walling said some women only ever had an orgasm when they were on holiday because that was the only time they were ever sufficiently relaxed.

He said: "I saw one couple who had an absolutely wonderful relationship, but who could not conceive.

"When I visited them they had charts all over the wall detailing how many orgasms the man had achieved, and how long they had lasted.

"We got rid of all the charts and the woman got pregnant."