When it comes to sex, there are certain pearls of wisdom that many of us take as gospel. One is that lovemaking is risky after a heart attack - because it might trigger another one.
However, a study published last month in the Journal of the American College of Cardiology suggests this worry is misplaced. German researchers tracked more than 500 heart attack survivors and found having sex carries no greater risk of triggering another heart attack than climbing the stairs or taking a brisk walk.
While some 100 heart attacks, strokes and other cardiac episodes occurred in those studied over the next ten years, no link was found between these and sexual activity - the scientists found that only 0.7 per cent of those studied had had sex in the hour before their heart attack.
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However, the situation might be different for those who are being unfaithful. A 2012 study published in the Journal of Sexual Medicine found men were more likely to experience 'sudden coital death' - a fatal heart attack during sex - while having sex with someone other than their wife.
One theory is the adrenaline-stimulating effects of sex may be heightened with a new partner, placing more strain on the heart, suggests Dr Mark Knapton, associate medical director of the British Heart Foundation. 'It's a bit like the excitement of watching a football match, which has also been reported to lead to increased numbers of calls to emergency services because of cardiac problems,' he says.
Here, we put other popular ideas about sex and health to the test...
Will having sex keep you slim?
If you thought sex helped to burn off calories, think again. In fact, the average session of sexual activity lasts six minutes and uses up a paltry 21 calories, according to a paper published in the New England Journal of Medicine in 2013.
Can a headstand help you get pregnant?
It's thought standing on your head after sex may aid conception, helping sperm to swim without battling the effects of gravity. But you don't have to go that far - just lying down may help.
A randomised controlled trial published in the British Medical Journal in 2009 looked at 400 couples having fertility treatment. It found that the women who lay flat for 15 minutes after sex had a 50 per cent greater chance of becoming pregnant than those who got up immediately and walked around.
The same principle may hold true for conceiving after sex, says Dr Gabrielle Downey, a consultant gynaecologist and obstetrician at BMI The Priory Hospital, in Birmingham. She suggests lying down with the pelvis slightly tilted upwards after sex may help.
'The uterus is bent slightly forwards, so if you tilt your pelvis upwards a little - say, by putting a pillow under your lower back - the sperm don't have to swim as strongly because they're not going against mucus and gravity,' says Dr Downey. 'That means they are effectively travelling downhill.'
She adds that 15 minutes is the average time it takes for sperm to reach the fallopian tubes to fertilise an egg, but suggests lying down for half an hour 'to ensure this process has enough time to happen'.
After that, gravity isn't quite as important to sperm's movements, as the fallopian tubes perform rhythmical movements that help the movement of sperm to the egg.
Does sex give you a headache?
Sex can sometimes cause headaches because of tension in the neck and surrounding muscles experienced during sex and orgasm.
These 'primary sex headaches' occur in around 1 per cent of the population, says Dr Katy Munro, a headache specialist at the National Migraine Centre in London - 'but they are more common in men'.
However, orgasm may actually help the pain of a headache - cancelling out that age-old 'Not tonight, darling...' excuse.
In research published in 2013, neurologists from the University of Munster, Germany, found that more than half of migraine sufferers who had sex during an episode experienced an improvement in symptoms - and one in five of the 800 migraine sufferers finished with no pain at all, a difference particularly seen in men.
The researchers suggested sex triggered the release of endorphins, the body's natural painkillers.
Can you conceive on your period?
Contrary to popular belief, a healthy woman between the ages of 15 and 50 can get pregnant at any time, says Dr Gillian Lockwood, a consultant obstetrician and gynaecologist and medical director of Midland Fertility Services.
A woman is most fertile in the three to four days around days 18 or 19 of her cycle (day one being the first day of the period), she says.
However, a healthy egg released during ovulation has a lifespan of possibly around four or five days, and healthy sperm have fertilising capacity for up to four days after being released, she adds. 'This is why we hear about lots of women who've thought: “It's near the end of my period, I'm perfectly safe” - but actually, they get pregnant.'
Furthermore, after having a baby the chances of becoming pregnant during your periods increase, says Dr Downey. This is because the hormones that stimulate the ovaries to make eggs return to normal around six weeks after a birth and sometimes reach even higher levels. This can mean you're more fertile at any point in your cycle.
'The effect is so strong, it's almost like having fertility treatment - so don't use having your period as contraception,' warns Dr Downey.
Will using the loo cut infection risk?
Around half of all women will develop a urinary tract infection (UTI) - where the bladder becomes infected, causing symptoms such as pain or a burning sensation during urination, a frequent need to urinate and cloudy urine - at least once in their lifetime. One in 2,000 men will develop one each year.
Sex can be the culprit, as harmful bacteria can be transferred from the bowel into the urethra, causing an infection (women are more at risk, as they have a shorter urethra).
Sex is a common cause of UTIs in women in their 20s, says Dr Zaki Almallah, a consultant urologist at BMI The Edgbaston Hospital. But research suggests post-menopausal women are at risk, too - the lack of lubrication and acidity in the vagina (due to a drop in the hormone oestrogen) can lead to an increased risk of irritation and infection.
To prevent this, many women use the loo before and after sex - but experts say this is only half-useful.
Dr David Kaufman, a New York-based urologist, previously claimed that urinating before sex 'is the number one cause of post-coital urinary tract infections'.
He suggests if you refrain from urinating before sex, it means you will have enough urine stored in the bladder to create a strong stream, so it's more likely any bacteria that does enter will be flushed out again when you go to the loo afterwards.
Bacteria that cause UTIs usually live on the perineum (the area between the bottom and urethra) and sex can transport these into the bladder through the urethra.
While Mr Almallah says there's no real evidence for this theory, he suggests there's no need to urinate before sex if you don't need to go, as it will have no effect on preventing a post-sex infection. However, going afterwards can help.
Should you avoid sex with a bad back?
Four out of five people experience at least one episode of debilitating lower back pain, and up to 84 per cent of men and 73 per cent of women say it affects their sex life, according to Canadian statistics.
Indeed, many think sex will make their symptoms worse, but this isn't the case, says physiotherapist Tim Allardyce of Surrey Physio. The key may be choosing certain positions, suggests research published last year in the journal Spine.
Women with lower back pain that is made worse when they arch their backs experienced less pain in the missionary position, the researchers found. 'Lying flat on the back with the knees bent and feet flat can be safe and comfortable for the woman,' says Tim Allardyce. For men, kneeling may help.
But while there's no reason those with back pain need to avoid sex, some things are just asking for trouble, he adds. 'When people start doing things like having sex in cars and showers, they may find themselves aggravating an existing back problem or creating a new one.'
Can sex bring on an early labour?
It is possible for sex to induce labour, but the pregnancy needs to be at a stage where the cervix is ready for it, says Dr Downey.
Semen contains prostaglandins - substances that have hormone-like effects, softening the cervix and stimulating contractions. Synthetic forms of these are used to induce labour, which is why it's possible sex may hasten labour. But only if you're close to your due date.
'Your cervix has to widen and be receptive to prostaglandins, and this only happens near the end of your pregnancy,' says Dr Downey. 'If your cervix isn't ready, having sex at 20 weeks won't trigger early labour.'
But it may induce Braxton Hicks contractions - tightening feelings in the uterus often felt in the second or third trimester and sometimes mistaken for labour pains.
PS: Why sex is good for heart patients
Generally, sex is important for heart patients, says Dr Knapton.
'Regular sex is part of returning to a healthy, fully active life and, in that sense, it's as helpful psychologically in boosting people's self-confidence and a sense of safety in their own bodies.' But, he adds, wait four to six weeks after a heart attack before resuming sexual activity to ensure the heart has healed.
Those with significant heart failure or a history of severe angina might experience breathlessness that could limit their sexual activity and should talk to their doctor.
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