All Boys or All Girls? A Study Reveals Why It Happens

Every expectant parent asks this question at one point or another. Whether it’s when that second line first appeared on the pregnancy test, or when that preggy bump first appeared, we couldn’t stop ourselves from wondering and wishing for the gender we want.

Of course, as Asians, we have our own beliefs when it comes to predicting or even increasing our chances of getting the gender we want. Some believe that the position plays a factor, while timing is key for others, and then there’s the Chinese gender calendar, which considers the parents’ age and date of conception. But can we tip the odds to get the gender we want? Or is contraception similar to a coin toss, where we get a 50/50 shot of getting what we want? But what if the toss is not so fair at all?

New research says that the odds might not be as fair as they seem: “Sex at conception… has long been considered a textbook example of a simple binomial distribution, implying that each fertilization event is random and independent, like a coin toss.” Instead, the research suggests that some families may be “wired” for boys or girls: “Some families consistently have offspring of only one sex, raising questions about whether sex at birth is truly random.”

The Science Advances Research Article tracked 58,007 women, covering 146,064 births from 1956 to 2015, using the Nurses’ Health Studies. And one of the study’s key findings is that offspring sex is not equally likely between boys and girls within every family. Instead, births align better with a beta-binomial model, which means that some families are statistically more likely to produce same-sex children, likening it more to a weighted coin toss.

To put it simply: “Each family may have a unique probability of male or female births.”

Furthermore, larger families showed more same-sex clusters. Statistically, families with three boys had a 61% chance of another boy, while families with three girls had a 58% chance of another girl, disproving the idea that each birth is a standalone 50/50 shot.

The study also cited several reasons why parents might have a higher chance of conceiving kids of the same gender. One is the mother’s age: “Older age at first birth was associated with higher odds of producing only males or only females.” Another is the discovery of specific maternal genetic markers associated with families having all daughters or sons.

It is also worth noting that the study team considered parental preferences by excluding the last child born to every woman participating in the study, to account for parents’ decision to stop having children once they reach their desired family size.For example, if a family of two boys tried for a third baby, hoping for a girl, researchers did not include the third child in the study analysis. However, even if the study included the last children of older women, the sex predictions remained the same.

Statistically, both World Bank data and the Philippine Statistics Authority (PSA) show that the sex ratio at birth in the country is around 1.08 boys per girl, or a ratio of 108 boys for every 100 girls, which is slightly higher than the global ratio of 1.05-1.07, but still within the typical variation.

This means that on a national level, we’re still close to a 50/50 split. So if a Filipino couple already has two or three boys (or girls), the odds for the next child might tilt slightly, but still not enough to guarantee anything.

Ultimately, yes, biology, behavior, and even parents’ age can affect your baby’s gender, but not in any deterministic, controllable way. It is still mainly by chance, with some unpredictably interesting twists at the family level.

Here’s how you can predict your baby’s gender using the Chinese Gender Calendar.