Medical myth: You can control the sex of your baby

by Monique Robinson, The Conversation
Timing sex around ovulation doesn’t change the odds of having a boy or girl. Credit: shutterstock.com

Despite most parents ultimately just wishing for a healthy baby, there are many cultural and social factors that can drive the desire for a baby of a particular sex.

The for sex selection of has existed in Australia for many years, but such an option is only available for medical reasons, such as sex-linked chromosomal disorder.

This leaves parents who do have a gender preference looking for natural ways of predetermining the sex of their baby.

In the 1960s, the idea that timing sex around can tip the odds in favour of a girl or a boy was popularised by Shettles and Rorvik in the best-selling book How to Choose the Sex of Your Baby.

Alongside thinking about the pH status of the reproductive tract, Shettles' idea was that Y sperm (leading to male babies) swim faster than X sperm (leading to female babies), therefore if sex is timed close to ovulation they will arrive at the egg first.

However, Y sperm live fast and die young. If sex occurs a number of days before ovulation, the Y sperm die off before they reach the egg, maximising the chances for X sperm to achieve fertilisation.

Leaving aside the quandary raised by conceiving opposite-sex fraternal twins via this method, what does the science say on whether timing sex can result in a shift in the gender ratio?

Two studies in the 1970s found very small shifts, with sex close to ovulation more likely to result in girls and sex on either side of ovulation more likely to result in boys. While rejecting Shettles' theory, these studies did find some influence of timing (albeit small).

Since then, the evidence has been mixed, but leans towards disputing any effect of timing on . Perhaps the most high profile study, published by American Allen Wilcox, found no evidence to suggest that the timing of sex around ovulation led to a significant change in the sex ratio of resulting babies.

If anything, again they found some minor support contradictory to Shettles' theory, with girls slightly more common when sex and ovulation were close together.

If timing sex around ovulation doesn't necessarily change the odds of having a boy or girl, what else might?

Some studies suggest that male conceptions are favoured in the midst of wars and conflicts. An interesting finding, given it's a time when male mortality rates are high.

Contradicting this, other research finds that extreme stress can lead to more female births. The cause of this is unknown but may be related to the increased fragility of Y during stressful times or general hormonal changes that favour females when times are tough.

More research has focused on maternal diet pre-conception to predict changes in the . Mothers who ate cereal for breakfast were more likely to have boys in one study. Another found that a low-salt, high-calcium diet favoured girls.

I was ecstatic to find that a respected Swedish pregnancy researcher conducted a scientific study of the accuracy of the much-googled Chinese lunar calendar sex-prediction method, based on an ancient chart "buried in a tomb for 700 years" but conveniently now available online.

Alas, planning your conception based on your Chinese lunar age and the month of conception turns out to be no more accurate than flipping a coin.

Perhaps the most interesting study I've come across on this topic is one examining the offspring of the 2009 Forbes 400 Richest Americans list.

In this study of billionaires (Bill Gates is at the top), men who inherited their money (heirs) were more likely to have sons than both self-made billionaires and the general population.

Heiresses were more likely to have female children than heirs, self-made billionaires and the general population. There were too few female self-made billionaires (just three) to be included.

Harking back to evolutionary theory where higher parental resources lead to more male births, the author suggested that wealth without stress led to sons. He theorised that self-made billionaires were under more stress than heirs, plus due to the years required for empire building, they may have children prior to achieving their wealth.

So, scheduling sex to coincide with ovulation may not give you the little Mary-Jane or Thomas you were looking for. Moving to a war zone or starting your day with Special K might tip the scales towards Thomas, but if the war zone is stressful or you add too much high-calcium milk to that cereal, you're sending the odds back towards Mary-Jane.

Chances are, the moment you hold your new baby for the first time, it won't matter anyway.

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