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
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. Boys give you a round tummy, girls, a football-shaped bump. 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.)
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?
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:
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.”
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.
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.
* Alexopoulos, E C and Alamanos, Y. “Secondary Sex Ratio in Greece: evidence of an influence by father's occupational exposure.” Human Reproduction 2007 Nov;22(11):2999-3001.
* Benenson, Joyce, Philippoussis, Maria and Leeb, Rebecca. “Sex Differences in Neonates' Cuddliness.” The Journal of Genetic Psychology: Research and Theory
on Human Development, 160:3, 332-342
* Bernardes, J et al. “Sex differences in linear and complex fetal heart rate dynamics of normal and acidemic fetuses in the minutes preceding delivery.” Journal of Perinatal Medicine 2009;37(2):168-76.
* Boatella-Costa, E et al. “Behavioral gender differences in the neonatal period according to the Brazelton scale.” Early Human Development (2007), 83(2):91-97.
* Connelan, Jennifer, Baron-Cohen, Simon, et al. “Sex differences in human neonatal social perception.” Infant Behavior & Development 23 (2000) 113–118
* Di Renzo, Gian Carlo, Rosati Alessia et al. “Does fetal sex affect pregnancy outcome?” Gender Medicine March 2007 4:1 19-30.
* Fan, Lynn Y and Jacobsen, Kathryn H. “Risk factors for hyperemesis gravidarum.” Current Women's Health Reviews 6:1 309-317.
* Goodman, Brenda, MA. “Testosterone May Delay Boys' Speech Development.” http://www.webmd.com/parenting/baby/news/20120125/testosterone-may-delay-boys-speech-development , sourced 25 February 2013.
* Guinsberg. “Differences in pain expression between male and female newborn infants.” Pain 85:1-2 127-133.
* Hansen, Dorthe et al. “Severe periconceptional life events and the sex ratio in offspring: follow up study based on five national registers.” British Medical Journal 1999;319:548.
* Ismail, Dr Saad Ramzi. “The Relationship Between Placental Location and Fetal Gender.” http://hcp.obgyn.net/fetal-monitoring/content/article/1760982/1878451 , sourced 16 February 2013.
* Jedrychowski, Wieslaw et al. “Gender differences in fetal growth of newborns exposed prenatally to airborne fine particulate matter.” Environmental Research 109:4 May 2009, 447–456.
* Khoury, M J, Erickson, J D and James, L M. “Paternal effects on the human sex ratio at birth: evidence from interracial crosses.” American Journal of Human Genetics. 1984 September; 36(5): 1103–1111.
* Little, B B, Rigsby, C H and Little, L R. “Pilot and astronaut offspring: possible G-force effects on human sex ratio.” Aviation and Space Environmental Medicine. 1987 Jul;58(7):707-9.
* Morokuma, A et al. “Fetal habituation correlates with functional brain development
.” Behavioural Brain Research 153 (2004) 459–463
* Nagy, E., Kompagne, H., Orvos, H. and Pal, A. (2007). “Gender-related differences in neonatal imitation.” Infant and Child Development, 16: 267–276.
* Paul, Marla. “Sex Differences in Fetal Alcohol Syndrome.” http://blog.womenshealth.northwestern.edu/2011/04/sex-differences-in-fetal-alcohol-syndrome/, sourced 20 February 2013.
* Roberts, C L, Algert, CS et al. “Small fetal size: a risk factor for breech birth at term.” International Journal of Gynaecology and Obstetrics 1999 Oct;67(1):1-8.
* Tamimi, Rulla M, et al. “Average energy intake among pregnant women carrying a boy compared with a girl.” British Medical Journal. 2003; 326:1245
* Wilcox, Allen J, et al. “Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby”. New England Journal of Medicine 1995; 333:1517-1521.
* Zukerman, Wendy. “Male fetuses ignore their stressed-out mothers.” http://www.newscientist.com/article/dn19195-male-fetuses-ignore-their-stressedout-mothers.html , sourced 18 February 2013.