So does it really work?

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There's still no strong scientific evidence to support Shettles' claims and a recent computer analysis of sperm failed to find the supposed differences between male and female sperm.

'Studies are few and far between,' says fertility expert Laurence Shaw of the Bridge Fertility Clinic in London. 'I suspect 75% is a bit optimistic. But there's evidence that pregnancies due to failure of the Pill are more likely to be female,' he says. 'If Shettles is right, this might be because progestagens in the Pill thicken cervical mucus. This makes the journey harder, supposedly favouring sperm with X chromosomes, which Shettles says are stronger.'

Choose your diet

A study at the University of Missouri found that mice gave birth to more females if they were fed diets low in saturated fats and high in carbohydrates. Another theory suggests that if you eat lots of calcium and magnesium-rich foods, such as dairy, spinach and wholemeal bread while you're trying to conceive, you're more likely to have a girl. This is based on the idea that there are four mineral salts in the body - calcium, magnesium, sodium and potassium - which can affect the way an egg is fertilised.

If a boy is desired, sodium and potassium-rich foods, such as meat, sausages, rice, pasta, beans and salted meat products, are the key minerals. But food high in salt is certainly not recommended. 'I wouldn't advise anyone to cut specific minerals from their diet while trying to conceive, as they might deprive the growing foetus of essential nutrients,' says nutritionist Victoria Greaves. 'You certainly shouldn't exceed the recommended daily intake of 2.5g sodium, or 6g salt.'

'After having two boys I was happy to adopt a specific diet when trying for number three,' says Beth Collins, 30, mum to Jake, 4, Matthew 2, and Rachel, 9 weeks. 'I really wanted my next baby to a girl. I read all the books and worked out the 'girly' times of the month, and informed my husband in a light-hearted way that we should give it a go. We waited until the birth to find out the sex, and I was incredibly nervous. I was so geared up for another boy, I was in absolute shock when Rachel came out!'