Boy or Girl?

We're keeping the sex of Science Baby a surprise. But what's the most likely outcome?

First, Science Dad asked the panel of "old wives".

• Dangling a pendant over the bump didn't seem to be a very robust indicator, and probably more likely a measure of Coriolis than conception. 
• Carrying low is presumably just a matter of orientation, not ovulation. 
• Cravings for salty versus sweet food certainly just dietary, and definitely not a factor in due-datery.

Old wives, you are full of lies! See a full list here.

I don't think the 'Mayan gender predictor' (here) is any more credible.

Ok, then what does Science say?

Science Mum (like all women) has two X chromosomes. Her eggs contains half of her genes, and so her eggs all contain an X chromosome. Science Dad (like all men) has both an X and a Y chromosome. Half of his sperm are X and the other half Y.

(from here)

Therefore, whether Science Baby is a boy or a girl comes down to which of Science Dad's sperm meets Science Mum's egg. Half of his sperm are "female" (X) and the other half "male" (Y) and when combined with the egg will produce a cell with two X chromosomes, or an X and Y respectively.

What conditions might increase the likelihood of the male sperm fertilising the egg rather than the female sperm?

• Male sperm move faster but don't survive as long as female sperm. A boy is more likely if you conceive close to when you ovulate.
• Therefore regular attempts to conceive will increase the chance of it being a boy.
• For couples who live together, 51.5 per cent of births are male.
• The longer you are married, the more likely it will be a girl (no prizes for guessing why).

• Male sperm and embryos are more 'fragile', and so are statistically less likely to survive.
• More boys are conceived in early summer, and more girls in winter. perhaps due to increased level of illness in winter.
• More girls are born in the tropics than anywhere else in the world. perhaps due to high temperatures burning dad's balls, or maybe due to the effects of melatonin on reproduction.
• The average calorie intake for women who had boys was 2,413. The average calorie intake for women who had girls was 2,283. Does this mean women need to be more nourished in order for a boy to survive?

• Mother's high stress levels lead to increases in both testosterone and cortisol, which leads to changes in the egg that makes it easier for male sperm to penetrate.
• More boys are born at times of stress (such as after a war).

• Severe pregnancy sickness is linked to female babies, due to high levels of oestrogen.

Check out this predictor on the babycentre. In reality, these factors probably tip the bias in favour of a boy or a girl by only a few percent. Unless you are planning to have hundreds of children, it probably won't be of much use when deciding what colour to paint the nursery*.

*although Science Dad doesn't believe nursery colour should reflect what sex organs the baby has. This is just a turn of phrase.