Online forums are filled with people sharing tips and success stories of methods to help conceive boys or girls. But are they just old wives tales or are some of them based on real science?
Here are a few popular methods hopeful parents-to-be use to try to get a baby of their preferred gender – and what an expert says about whether they could really work.
Timing sex around ovulation
Timing sex around ovulation has been the most popular gender swaying method since Dr Shettles came up with his theory in the sixties. He believed Y sperm (which helps create a boy) were faster swimmers but had lower stamina, while X sperm (to create a girl) were slower swimmers but survived longer. Shettles thought that conceiving a girl was done by having sex a few days before ovulation, while conceiving a boy was more likely when sex took place on the day of ovulation or shortly afterwards. Other techniques, such as the Billings Method, encourage couples to time sex in the same way.
Deeper sexual positions and female orgasm are also supposed to increase the chance of conceiving a boy by helping Y sperm move faster through the vagina.
What the expert says: Associate Professor Steve Robson from the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) agrees that Y sperm are smaller, but they don't necessarily move faster. "The movement of different sperm is incredibly variable. I've never seen real evidence that these old-fashioned techniques work," he says.
Credibility: There's no guarantee timing sex will work. Using this method to try and conceive a girl might mean some couples actually miss ovulation.
Changing the pH
Certain gender swaying methods – including the Shettles method and the 0 +12 method – focus on how pH levels within the woman's body affect cervical mucus (CM). A more alkaline environment is supposed to be more favourable for Y sperm, while X sperm are supposed to be more resilient in an acidic environment.
The 0 +12 method is the opposite of the Shettles method, because it recommends sex takes place 8-20 hours after ovulation to conceive girls. Both methods involve women using acidic or alkaline douches, but these aren't recommended by doctors as they can cause infections. Modern twists on douching include taking antihistamines during ovulation to 'dry up' CM, to try and make it easier for X sperm to thrive, or using Pre-seed lubricant to neutralise the vagina for Y sperm.
Other methods, such as the French Gender Diet, recommend eating certain foods and taking supplements that are high in certain nutrients to alter the pH of CM. This includes calcium, cranberry and magnesium for conceiving girls, and potassium and sodium for conceiving boys.
What the expert says: Professor Robson says the pH of CM regularly changes, but this is just the natural cycle of the body, and not necessarily because of diet or other factors.
"Mucus is generally acidic. The alkalinity of pH during ovulation combined with semen helps to make things a little more sperm-friendly because neither X or Y sperm will survive in a really acidic environment."
Credibility: While this is a little credible as it is possible to change the pH of CM, the risk may not be worth it – if it becomes too acidic it could actually make it harder to fall pregnant.
Frequency of sex
Shettles believed men should ejaculate more frequently when trying for a girl, and abstain for up to 5 days before trying for a boy. This is because he believed that men with lower sperm counts were more likely to conceive girls, while men with higher sperm counts usually conceived boys.
Couples who use the 0+12 method to try for a boy are also supposed to avoid sex for a week before ovulation to prevent the alkalinity of semen, making CM more hospitable for Y sperm.
What the expert says: Professor Robson says abstaining from sex won't influence sperm production. "I'm not sure how such myths arise. Reductions in sperm concentration can be caused by many factors, including things such as the man having a fever," he says. "I always advise couples trying for pregnancy to have intercourse at least every second night but every night is even better."
Negative and positive ions
Ions are charged particles in the air; some are charged negatively, and some are charged positively. Negative ions are said to be produced in natural environments, while positive ions are more often found in environments with less air flow and artificial lighting.
Some scientists, such as Dr Patrick Schoun, have suggested that negative ions help sway for girl babies and positive ions for boys. Bracelets and air purifiers that supposedly attract negative ions are available to help conceive girls.
What the expert says: Professor Robson is dubious. "Ions are part of the body but trying to attribute effects to one ion over another is very strange, because the concentrations change from minute to minute."
Credibility: Not much at all.
Scientists Trivers and Willard believed that different levels of hormones in a female's body at the time of conception played a part in gender selection.
High levels of gonadotropins and progesterone and low levels of testosterone apparently help to sway for girls, while high levels of oestrogen and testosterone have been associated with helping to conceive boys. Some women try to consume products that can affect these hormones, such as taking soy to encourage oestrogen production or eating more red meat to boost testosterone.
Taking certain medications can also affect hormone levels at conception, as well as a woman's weight. Recently a study found high levels of glucose in mums-to-be resulted meant they were more likely to have a boy, and women who ate fewer kilojoules a day had more girls.
What the expert says: Professor Robson says the body might favour one sex over another in certain circumstances. "There are many different hormones in the body that affect fertility. These are commonly affected by factors such as stress and general health – some research done in the UK a few years ago found that when women were stressed they were more likely to conceive a girl.," he says. "How this might occur isn't clear, but I think it's worth a further look."
Credibility: There definitely seems to be some credibility to this theory, but more research is needed.
Trying to conceive? Join others who know what you're goig through in the Essential Baby forum.