Can old wives' tales tell me my baby's sex?

This article is taken from a series of articles titled bumpology. The article states:"Bumpology is our weekly column on the science behind pregnancy, written by our reporter whose own bump is growing larger by the day".

Deciding whether or not to find out the sex of our unborn child was a dilemma that plagued us for weeks. Would it spoil the surprise? Even without getting a definitive ultrasound examination to tell us the sex, there are any number of folklore prediction methods – but is there any science behind them?

Old wives' tale 1: Bad morning sickness means you're having a boy

I didn't suffer morning sickness, so according to this rule, I should be expecting a girl.

The opposite is true, in fact: mothers with severe pregnancy sickness are more likely to have a girl. In 1999, researchers analysed records of 8186 women admitted to hospital in Sweden with pregnancy sickness and found that 44.3 per cent of them gave birth to boys, versus 51.4 per cent of the general population (The Lancet, DOI: 10.1016/s0140-6736(99)04239-7). They blame higher levels of the pregnancy hormone human chorionic gonadotropin (hCG) in women carrying girls.

Curiously, Henrik Toft Sørensen of Aarhus and Aalborg University Hospitals in Denmark wrote to The Lancet the following month claiming that the sex ratio also seemed to be influenced by marital status, with fewer boys born to single mothers.

In a separate study, he also found that women with severe pregnancy sickness were more likely to give birth to girls; the correlation was even stronger in single mothers. Among women with severe sickness, 46.5 per cent gave birth to boys, compared with 51 per cent of other women. Within the pregnancy sickness group, 40 per cent of women who lived alone had boys, against 45 per cent of women who lived with their partner (The Lancet, DOI: 10.1016/s0140-6736(05)74029-0).

Old wives' tale 2: Fetal heart rate predicts the baby's sex

According to this rule – it's been around for 30 years, so I class it as an old wives' tale – if your fetus's heart rate is above 140 beats per minute (bpm) you're expecting a girl, and if it's below you're expecting a boy. I've had the pleasure of listening to my baby's heart rate twice in the past fortnight, and the last time it was 130 bpm – so I should be expecting a boy.

However, fetal heart rate tends to decrease as pregnancy progresses, from 170 to 200 bpm at 8 to 10 weeks down to 120 to 160 bpm by mid-pregnancy. And according to published studies there seems to be little difference between boys and girls, at least during early pregnancy.

In one study, researchers used ultrasound to measure fetal heart rate in 477 fetuses before 14 weeks of pregnancy. The average heart rate of girls was 151.7 bpm, while for boys it was 154.9 bpm – not a large enough difference to be statistically significant (Fetal Diagnosis and Therapy, DOI: 10.1159/000089065).

The only time when a difference has been detected is during labour itself, when female babies seem to have faster heart rates than males; the reason is unknown.

Old wives' tale 3: Weird food cravings = a boy

One friend told me that my aversion to lettuce means I'm going to have a boy. I couldn't find any science to support this, but there is some evidence from a Boston hospital study that found expectant mothers have bigger appetites on average if they are carrying a boy rather than a girl.

I'm definitely hungry all the time, but that might just be because of pregnancy in general. The problem is: how do you know what constitutes a big increase in appetite? Indeed, the authors of the Boston study admit that the difference in appetite is not striking enough to predict the sex of a baby with accuracy.

Old wives' tale 4: Women "just know"

That mysterious thing called "female intuition" claims power over many things, including being able to predict the sex of unborn children. Personally, I suspect it's a cunning tactic for dismissing ridiculous baby names suggested by the male partner. Having said that, I have been convinced I'm having a girl for some weeks.

In 1998, researchers at the University of Tucson in Arizona asked 108 pregnant women to predict the sex of their baby. Seventy-five of these women claimed to have an intuition about it – as either a gut feeling or a dream – and of these women, 60 per cent guessed correctly. When women who had a preference for one sex over the other were removed from this sample, however, the 48 women that remained guessed correctly 71 per cent of the time. Unfortunately, the study was never published, making it hard to scrutinise the details.

As well as all the tales above, of particular interest to Bumpology is the folklore surrounding the shape of the bump and its relation to the baby's sex (Birth, DOI: 10.1046/j.1523-536x.1999.00172.x). But predictions from the bump are no clearer than the other tales, and we decided to plump for a method of sex prediction that's 95 per cent certain. We asked the sonographer to tell us if our baby was going to be a boy or a girl during our 21-week scan last week.

All the old wives' tales got it wrong, bar one: female intuition. I'm happy to say that we're expecting a little girl.
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References:

Geddes, Linda. 2010. "Can old wives' tales tell me my baby's sex?". New Scientist. Posted: April 27, 2010. Available online: http://www.newscientist.com/article/dn18827-bumpology-can-old-wives-tales-tell-me-my-babys-sex.html