Most popular methods
TIMING OF SEX
What's the theory?
Male sperm are lighter in weight and swim faster than female ones so are more likely to reach the egg. Having sex on the day of ovulation should increase the chances of conceiving a boy.
Female sperm are heavier and live longer so having sex a few days before ovulation and then abstaining (while the male sperm die) should increase the chances of conceiving a girl.
Dr Landrum Shettles developed this theory in the US in the 1960s. It's complemented by the Billings' Method, which is about timing sex around the consistency of vaginal mucus (when it's thin and clear for boys, thicker and sticker for girls). Couples using this method tend to have boys more than girls.
What's the success rate?
Advocates claim 75% but there's only a tiny amount of medical evidence in the UK to support it.
Confusingly, another theory exists based on observations that couples that didn't live together before conceiving had more girls. They were thought to be less likely to have sex at the time of ovulation, backing up the idea that conceiving four or five days before ovulation was more likely to produce a boy.
SEXUAL POSITION
What's the theory?
Male sperm are more vulnerable and the vagina becomes less acidic closer to the cervix. So if penetration is deep (woman-on-top or man-behind positions), the male sperm will enter a more alkaline environment and are more likely to survive the short journey.
Shallow penetration should favour your chances of conceiving a girl: the slower female sperm are hardy enough to withstand the acidic conditions at the entrance to the vagina so could make it through to the egg after the male sperm have died off.
What's the success rate?
Not high, since the theory fails to take into account that the difference in lifespan between male and female sperm is a matter of days, not minutes.
Medical sceptics say that force of ejaculation and physics of sexual position don't play a part in getting the sperm to their destination more quickly ? that's determined by the chemistry of cervical mucus.
MALE SEXUAL ABSTINENCE
What's the theory?
Abstaining for a few days before making love increases your chances of conceiving a boy. The greater the volume of sperm, the more male sperm should be present, so theoretically there's a greater chance that a male sperm survives and fertilises the egg before a female sperm can reach it.
What's the success rate?
Not high. Sceptics say that with more sperm, the chances of you getting pregnant may be higher, but there's no gender factor.
CHANGING YOUR DIET
What's the theory?
The minerals in your diet could affect your metabolism and the environment of your eggs.
- To improve conditions for male sperm you need potassium (in meat, bananas, apricots and celery).
- To improve conditions for female sperm you need more magnesium-rich food (in nuts, soya beans and leafy green vegetables).
What's the success rate?
There's no scientific data on humans to support the potassium or magnesium theories, although experts say diet does seem to play some part in gender selection.
Historical events have shown that famine can result in more boys being born but also that women weighing less than 54.5 kilos are more likely to have girls. This could mean that the type of food and nutrients available to those people played a part, or that calorie intake has an effect - but a low-calorie diet has a very different effect to malnutrition.
TIMING OF ORGASM
What's the theory?
If you reach orgasm before your partner, your vagina becomes more alkaline, which creates a better environment for male sperm. Female sperm survive better in the naturally acidic environment of your vagina, so climaxing first would favour the conception of boys.
What's the success rate?
Little. Prolonged foreplay has the same effect of increasing vaginal secretions, which could be just as effective at reducing its acidity ? but no one has suggested female foreplay as a method for conceiving boys.
DOUCHING
What's the theory?
Male sperm favour alkaline conditions, so douching (flooding the vagina via a syringe) with a dilute alkaline solution, such as sodium bicarbonate (baking soda) before intercourse increases the chances of conceiving a boy.
What's the success rate?
There's a very small amount of medical evidence that backs up this theory, but some experts warn women against putting sodium bicarbonate into their vagina because they may not know how diluted to make it, and in any case, the acid in the vagina is there for necessary protection against infections like thrush.