The real difference between boys & girls

What part does gender play before your little one is born? Sarah Tennant discovers the real differences between boys and girls.

We're obsessed with gender identity in babies. We love to point out the first proof of our baby's inherent masculinity or femininity — aha, she likes shoes! She must be a real girl! Look at him all excited about that truck, and with no prompting — he just knows! And we all know the father who won't let his newborn son wear a pink shirt, and the mother who brags that her daughter prefers bugs to Barbies.

We even assign gender to babies in the womb. Feel him kicking — he's going to be a football player!

But what if we didn't know? Is, say, a 22-week-old male foetus essentially interchangeable with a female one, genitals aside? Is a foetus a foetus, or does its sex make a difference?

Old wives’ tales about gender before baby is born

Throughout history, when knowing a baby's sex wasn't an option, numerous myths about “boy” pregnancies and “girl” pregnancies did the rounds. Some were purely light-hearted superstitions, like the notion that if a woman picks up a key by the thin end, she's having a girl, but others reflected a belief that boy and girl foetuses were physiologically different and affected their mothers’ bodies in different ways.

Many of these myths are still current. Girls steal their mother’s beauty (a useful one to know if you can't resist telling a pregnant woman she looks like hell). Girls make you moody. Boys make your face look “harder” and make you act more aggressively. Cravings for sweet foods indicate you're expecting a girl, sour and salty foods, a boy. And then there are a lot of urine-related ones, which we need not discuss here.

A more recent myth is that a baby's sex can be determined by its heart beat — over 140 beats a minute for a boy, under for a girl. (It’s nonsense, research shows the heartbeat varies according to the baby’s activity level. The only time boys’ heart rates differ noticeably from girls’ is during labour, when you’re about to conclusively find out the sex anyway.)

Which side of the womb does a baby boy lie?

Oddly enough, the one really well-supported, minimally invasive way to determine gender before the 20-week ultrasound sounds like the old-wifiest tale of them all. But a 10-year study of 5000 women found that, with 97% accuracy, boy babies implant on the right side of the uterus, and girls on the left. No one’s quite sure why but isn't that nifty?

Difference between boy and girl movement in pregnancy

Your baby will start to move as early as seven weeks along, making slow neck movements. They then add to their repertoire, with hiccups, thumb sucking, yawning, kicks, punches, and rolls. However, most Mums won’t feel much until the 16^th to 18^th week of pregnancy. There are a lot of variables that make a difference to how much you feel baby’s movements. If you’ve had a previous pregnancy, you may feel ‘the quickening’ from 13 weeks, while some first time Mums with a posterior uterus may not feel anything until 26 weeks.

The movements the baby makes are important to develop strong bones and joints, and you will definitely feel the baby getting stronger as he or she grows – sorry about your ribs, bladder and cervix, Mum. There’s no real established ‘normal’ number of movements during pregnancy, every baby and Mum is different.

To study the difference between baby boy and girl movement in utero, there are technical difficulties. The only way to truly measure which gender is more active in womb is in hospital, and only for short periods of time when hooked up to ultrasounds. That’s unsustainable to do 24/7, so the current research is based on short time periods.

So, who moves more, boy or girl?

There’s no clear science on who kicks more during pregnancy, boy or girl. There have been many studies that have found boys move around more than girl babies. The biggest difference between the movement of baby boy and girl in one study was that there were more leg movements in boys at all stages throughout pregnancy. However other studies found no correlation between gender and number of movements in utero. It’s possible there’s no difference between boy and girl movement, and science can’t guarantee anything 100% yet.

Who does strong kicks, a boy or girl?

Kicks are said to have a force of 2kg at 20 weeks, and that doubles to 4kg of force at 30 weeks. It decreases after that, as there is less room for the baby to move—no more getting a run-up at your bladder. Unfortunately – or perhaps, luckily, there seems to be no correlation between strength of kicks in boy or girl babies.

Difference between boy and girl bumps

A popular old wives’ tale is that if the Mum has a round, high bump that sticks out, it’s a boy. If the belly is rugby-ball shaped or spread around the middle, it’s supposedly a girl. Anecdotally, people confirm this- old Aunt Joan confirms that she had two babies and the boy was high and in front while the girl was low and spread out. But does science agree with the Aunties?

There are two main factors that come into play with the bump. Once is the size of the baby. On average, boy babies weigh more at birth than girl babies. But this difference of a few hundred grams is unlikely to make any real difference to the visible bump.

The second factor is the position of the uterus and the foetus in it. If the baby has his or her back facing outside, this makes the bump stick out more. This is totally unrelated to gender, so there is no way there’s a difference between a baby boy and baby girl bump.

While the bump shape is a myth, if you’re currently pregnant with a boy and are constantly hungry, there’s some science to that. The more weight a woman gains in pregnancy, the more likely the baby is to be a boy. Apparently the more testosterone floating around, the hungrier Mum is, and the more weight she puts on.

Who’s having boys?

Right from conception, boys and girls are different. To start with, an estimated 125 male babies are conceived for every 100 females. The reason is unclear. Some say “male” Y-bearing sperm are faster — a heavily disputed notion. Others say Y-bearing sperm are more numerous but studies of men who have fathered several children of the same sex show that the 50/50 ratio of X and Y-bearing sperm is pretty constant.

What's more, some demographic groups have a hard time fathering sons. If you think everyone has a 50/50 chance of a boy, consider these cases:

☙ A Scandinavian study found that babies born in spring were more likely to be female.

☙ A higher ratio of boys to girls occurs in warmer years. However, countries near the equator produce more girls than countries near the poles.

☙ Black men produce a higher ratio of daughters than white men do, while Asian men have the highest ratio of sons.

☙ Men with a disproportionate number of daughters include long-distance runners, fighter pilots, astronauts, farmers who apply pesticides, men occupationally exposed to certain chemicals and heavy metals and, according to one oddly specific Greek survey, ship's carpenters.

☙ Men who smoke heavily are more likely to father girls.

☙ More girls than boys are born in times of famine and national hardship, or after a woman has experienced a traumatic life event.

As one of six girls, I had a quick look to see if any studies showed pastors were more likely to produce girls. No dice, but Dad is an older father, another “risk factor” for a houseful of oestrogen.

In other words, there are plenty of reasons why boys might not run in the family. Some of them are still uncertain, but one common theme in the above studies is that boys are something of a fragile luxury item: able to grow and develop only if the conditions are just so. Too many pollutants, not enough food, and they simply won't make it, often succumbing very early on, before a woman even knows she’s pregnant. By birth, the ratio has dropped to 105 boys for every hundred girls.

And when the ratio of boys to girls drops in a population, overall fertility tends to drop as well. That's why researchers keep an eye on the secondary sex ratio (boys to girls at birth). If it dips, it may be a sign that a population’s reproductive systems have been compromised, often by environmental contaminants.

Girls, on the other hand, are easier to grow. Female foetuses seem perfectly content to hang out in sub-standard conditions. Averaging 100g lighter than males at birth, they require fewer nutrients to thrive and they're less susceptible to the effects of everything from Foetal Alcohol Syndrome to heavy air pollution.

The male risk factor

But it doesn't end there. Not only are boys the weaker sex, they cause more problems for Mum.

One meta-analysis of studies regarding foetal gender differences found:

“Women carrying male foetuses had higher rates of gestational diabetes mellitus, foetal macrosomia (big baby syndrome), failure to progress during the first and second stages of labour, cord prolapse, nuchal cord and true umbilical cord knots. Caesarean sections were also more frequent for male neonates.”

The studies' authors concluded: “Male sex is an independent risk factor for adverse pregnancy outcome.”

Thanks, boys.

Not that girls are little angels either. They're more likely to give you hyperemesis gravidarium (severe morning sickness), more likely to suffer twin-to-twin transfusion syndrome and, due to their smaller size, more likely to be breech.

However, while they may cause you to throw up for nine months straight, at least they'll feel your pain while you're doing it.

A study of the effects of the stress hormone cortisol on foetuses focused on asthmatic pregnant women, simply because high levels of cortisol are released during asthma attacks.

The researchers measured the cortisol levels in the babies’ cord blood, as well as examining the placentas for the expression of genes related to stress response.

It turned out that baby boys, exposed to the hormonal evidence of their mothers’ stress, serenely continued growing as normal. The baby girls, on the other hand, had higher than average levels of cortisol in their cord blood and a number of them had low birth weights, a natural response to living in a stressful environment.

As researcher Vicki Clifton put it, “Females are very sensitive to what’s happening in Mum’s body, but males just ignore it.”

But the point of the study wasn't to prove that male callousness starts in the womb. It sheds some light on what tactless NICU staff refer to as “Wimpy White Boy Syndrome”, the fact that male premmies, especially Caucasian ones, don't thrive as well as premature girls.

In particular, premature boys have more trouble with respiration. Women going into pre-term labour are given stress-related hormones to help the baby’s lungs mature quickly. Because boys tend to ignore stress hormones, their lungs don't respond as readily; so a 28-week baby boy may have gestationally “younger” lungs than a 28-week baby girl. Hopefully, Clifton's research will spark new methods for accelerating lung development in premmie boys.

But ignoring Mum's stress isn't the only way boys’ brains differ from that of girls.

Brain differences between boy and girl babies

Initially, male and female brains are physiologically identical — both flooded with oestrogen from the mother. But when boys’ testicles begin producing testosterone, their brains start to change. At birth, a boy’s brain is slightly larger than a girl’s, a difference which remains throughout life.

Gender-related learning differences — slight advantages at spatial reasoning for boys, language and communication for girls — can be traced back to the hormones flooding baby brains throughout gestation. Girls who are exposed to high levels of testosterone in the womb later exhibit more typically masculine learning and behaviour patterns.

For obvious reasons, foetal intelligence is difficult to gauge but one study at least supports the “girls rule, boys drool” line of thought. Researchers measured how long it took foetuses to cease being surprised every time a repeating noise occurred. (The first bark of a dog is startling. Ten barks later, we're used to it.) At 33 weeks, female foetuses got used to it quicker than males.

Once born, boys and girls demonstrate slight neuro-behavioural differences. Boys tend to be fussier, a fact that I was surprised to learn, as the truism in my group of friends is that boys are easier. But science says so, and who am I to question?

Girls, on the other hand, are slightly more alert, exhibit a stronger pain response (during the heel-prick test) and are quicker to imitate adults. In a 2007 study, researchers encouraged babies between three and 96 hours old to mimic a gesture — extending an index finger. The results?

“Although... the total number of movements was similar in boys and girls, girls showed more fine motor movements, a higher number of specific imitative gestures, responded faster during the imitation and showed a higher baseline heart rate during the experiment.”

Yet another found that newborn girls tended to prefer looking at a human (female) face than a mechanically-moving mobile. Boys preferred the mobile.

My favourite study on the subject of neonatal gender differences, however, has to be an adorable little experiment from The Journal of Genetic Psychology. Entitled (I kid you not) “Sex Differences in Neonates’ Cuddliness”, the study featured a man and a woman holding a number of newborn babies while being filmed.

Unaware of the sex of the babies, the cuddlers then ranked them on cuddliness, and the scores were examined for relevance to gender.

Shortly after, a highly educated scientist wrote the following sentence: “The mean cuddliness scores were higher for the female than for the male neonates.”

And now you know.

Sarah Tennant lives outside Hamilton with her husband, a son and a daughter who, back in their day, were both plenty cuddly.