Does Family History Determine Baby Gender?
By Keshia Sophia Roelofs
Dec 15, 2023
•
6 min read
KL
Reviewed by Kellie Leonard,
Biomedical Scientist / Medical Writer
In this article: 📝
Does gender run in families?
You’ve probably heard it before: “The Smiths always have boys.” But is there any scientific backing to the pattern, or is it just a roll of the dice? Studies have long explored if certain genetic factors might influence the sex ratio in families. And what’s become clear is that each conception is a new genetic event. Yes, there might be slight trends in some families, but it’s largely a 50/50 chance each time—emphasis on largely. According to the World Health Organisation, the baby sex ratio is more male-biased, with 105 boys born for every 100 girls So, is it hereditary to have a boy or girl? Well, as one study explains, we can’t say with certainty that having a boy or girl is inherited when very real outside cultural factors like son preference and sex-balancing of families muddy the details. In essence, it’s hard to know what’s natural. But one study has come pretty close (more on that lower down). 👇 Note: While used interchangeably, we’re talking about biological sex here, not gender identity, which is a separate and more personal journey. Biological sex refers to the biological and physiological characteristics that identify males or females—hormones, anatomy, genetics, etc. Gender is the social roles, expressions, expectations, and behaviors layered onto those identities—girls, boys, men, and women. Yes, you may be able to predict a boy or a girl, but how they see themselves in time may change.Does sperm determine gender?
Here’s where science has a clear answer. Absolutely, the father’s sperm plays a crucial role in determining a baby’s sex, and the process is a marvel of nature’s lottery. Each sperm cell is like a tiny messenger, carrying either an X or a Y chromosome. It’s this chromosome that pairs with the egg’s ever-constant X chromosome to decide the baby’s sex. A match of XX results in a girl, while XY means a boy is on the way. But there’s more to it. The fascinating part is how the sperm race towards the egg—it’s like a microscopic sprint where the prize is not just reaching the egg but shaping a new life’s identity. This chromosome-carrying sperm is nature’s way of flipping a coin, ensuring a near-equal chance for either gender at conception. So, are male genes more dominant? In the dance of chromosomes, the father’s sperm is indeed the deciding factor. But it still takes two to tango.Why do I only have boy babies?
If you find yourself surrounded by a band of boys at home, you might be wondering if there’s a reason beyond sheer luck. Scientifically, the chances of having a boy or a girl are roughly equal with each pregnancy. But a 2008 study researching 927 family trees dating back to the 1600s suggests that the father‘s family history may play a key role in determining a baby’s sex. The research showed that men who had more brothers were more likely to have sons, while those with more sisters often had more daughters. Researchers are pointing towards a mysterious gene that may be controlling whether a man’s sperm has more X or Y chromosomes. And it may be an attempt to keep the sex ratio nicely balanced—except after periods of war or large male casualties when there tends to be a period of baby boy booms. Fascinating, no? Interestingly, it’s much harder to predict in women (presumably because the egg’s chromosome never changes).Why do I keep having girl babies?
Similarly, if you’re the queen of the castle with a court of daughters, it might seem like more than just chance. Just like with boys, having multiple girls could be down to dad having more sisters and a mysterious gene working to keep the sex ratio balanced. But mom’s well-being may have an impact too. A study following 338 UK women trying to conceive (TTC) found that the 50% with higher cortisol levels were 75% less likely to have a boy. The impact of stressful large-scale events—like natural disasters—has long been shown to tip the scales of the sex ratio, but now research is also linking more intimately-felt stressors like childhood trauma with increased chances of having a girl. Honestly, there’s a lot we’re still learning about genetics, but the predominance of one gender in a family often does come down to random chance.If your first child is a boy, what will your second be?
So, your firstborn is a boy, and you’re wondering if you should start stocking up hand-me-downs for the next one? Well, the odds reset with each pregnancy—it’s still a near 50/50 chance for either sex Data from India would suggest a different story: that if your first is a boy, you’re less likely to have a girl (with a ratio of 182:100). And if you have a girl, you’re chances of having a boy decrease slightly. But this points more to a cultural preference for sons, with many parents no longer trying once they have their baby boy. On the other hand, a 2002 study found that the chances of your second child being a boy increased if the first-born daughter had a higher birth weight. The same couldn’t be said for the first-born son. Still, with a data pool of 227 healthy moms, it’s early days in this research. Really, for the most part, past pregnancies don’t set the stage for the next. Might as well just flip a coin.So, does family history determine baby gender
In the grand scheme of things, family history might offer a fun narrative at family reunions, but it doesn’t hold much sway over a baby’s gender. Each pregnancy is a unique mix of chromosomes, with a roughly equal chance of a boy or a girl. So, whether you come from a long line of girls or boys, remember—every new baby brings a fresh start, adding their own unique story to the family tapestry. Let the ring tests, old wive’s tales, and guessing games begin!Popular on the blog
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