Most mums-to-be would say their main concern is their baby's health, not its gender. But many women - and men - would still prefer one sex over the other.
Although sex selection is possible in IVF cases, current regulations say it's only allowed to rule out a medical condition that affects just one sex, such as haemophilia.
For the rest of us, the only way to help tip the balance is to try the natural methods - none of which is guaranteed. The most common are:
Dr Shettles' technique of having sex at particular times is the most popular method. For a girl: have sex regularly from the day your period ends until two to four days before ovulation - which occurs about 14 days before your next period is due - then stop. For a boy: abstain or use condoms until four days before ovulation, then abstain totally until 12 hours prior to ovulation, then have sex, just once.
The main theory is to eat lots of calcium-rich foods for a girl while cutting down on salt. For a boy, increase salty foods and red meat and cut down on dairy products. However, increasing salt and reducing calcium can lead to health problems during pregnancy, so consult your doctor before starting a pre-conception diet. Scientists have also found a correlation between a mother's eating habits around the time of conception and the sex of her baby. A high-calorie diet - and in particular eating cereal for breakfast - is believed to tip the probability towards having a boy.
To conceive a girl, Dr Shettles recommends the missionary position and shallow penetration. To conceive a boy, try rear entry with deep penetration.
If you're trying for a girl, hold off completely. If you're trying for a boy, try to orgasm before your partner. This increases the flow of alkaline secretions in the vagina, which is best for boy-producing sperm.
Finding out the sex of your unborn child:
There is a lot of folklore about predicting your babys sex, such as the way youre carrying your bump (high up means a boy, lower down means a girl) and hanging a needle or wedding ring over your bump on a piece of cotton to see which way it spins (left for a boy, right for a girl). Neither method has proven to be accurate.
The only effective ways of telling the sex are ultrasound scans (although even if its obvious, some hospitals still wont tell you) and invasive tests such as amniocentesis and CVS, but these will only be offered if theres a medical need.
For more information on male fertility, sex and relationships visit Men's Health