But the world’s largest ever study conducted on the subject has finally put the age-old theory to bed.
The findings were presented today at Europe’s biggest fertility conference in Helsinki, Finland.
Dutch researchers studied 500 couples.
Half of the women were assigned 15 minutes bed rest with their knees raised after artificial insemination.
The other half got up immediately.
But experts discovered that lying down afterwards had no positive effect whatsoever on the chances of pregnancy.
It suggests for the first time that lying down after sex to boost your chances of having a baby is pointless.
Fertility experts at the annual meeting of the European Society of Human Reproduction and Embryology said the study showed there was no reason to stay in bed for longer than it takes ‘to get your breath back’.
Reacting to the findings, Prof Allan Pacey, of the University of Sheffield , said he was not surprised by the results because it takes just five minutes for sperm cells to reach the fallopian tube, after which they can survive for several days in the womb.
Prof Pacey added: “By the time you have recovered your breath, and you’ve got out of bed to have a wee, the ones [sperm] that are going to do it [fertilise the egg], are in there already.”
British Fertility Society chairman Dr Adam Balen, Professor of Reproductive Medicine & Surgery at Leeds Teaching Hospitals, added: “What’s very unhelpful in all biology textbooks is the pictorial representation of the female anatomy.
“It shows that the womb and the vagina are in a straight, vertical line. In reality that is not the case.
“The vagina is tilted and the womb is then tilted relative to the vagina and fallopian tubes and the ovaries, so it is not just one passage.
“So when you get up, many women think that everything is just going to fall out but anatomically that is not going to happen.”
Experts also stressed there was just one post-coital act which would damage conception chances - smoking a cigarette afterwards.
Dr Balen said: “After you have had sex, do whatever you want but don’t smoke.”
The study by the VU University Medical Center Amsterdam involved 479 women undergoing artificial insemination.
Women who had 15 minutes bed rest afterwards achieved pregnancy rates of 32.2 per cent, while those who did not had higher rates of 40.3 per cent.
After the results were adjusted to account for differences between the groups, there was absolutely ‘no statistical difference’.
Lead researcher Joukje van Rijswijk said that the study had proved that lying down had ‘no positive effect on pregnancy rates’.
She added: “We believe our results in such a large randomised trial are solid, and sufficiently strong to render the recommendation for bed rest obsolete.”