Boy or girl? Chance of having a son or daughter is NOT 50/50, scientists say
For years, it was assumed that the chance of having a son or daughter also fit into this category.
But a shocking new study has revealed that this isn't actually the case.
Scientists from Harvard University have discovered that some women are much more likely to have children of only one sex.
In their study, the team analysed data from more than 58,000 mothers who had given birth to at least one child.
Their results revealed that maternal age plays a key role in determining the sex of the babies.
'The human sex ratio has long been of interest of biologists, statisticians, demographers, sociologists, and economists,' the team explained.
'Here, we showed that within each sibship size, sex at birth did not conform with a simple binomial distribution and identified a significant intramother correlation in offspring sex.'
The sex of a baby is determined by the combination of sex chromosomes inherited from the sperm and egg.
While the egg always carries an X chromosome, the sperm can either carry an X or a Y.
If the sperm carrying an X chromosome fertilises the egg, the embryo will be female, while the embryo will be male if it's a Y chromosome.
This has long been considered to be a 50/50 split.
However, in their new study, published in Science Advances, the scientists questioned whether this is really the case.
'Several coauthors, however, observed cases of friends, colleagues, first-degree relatives, or themselves that produce offspring of only one sex raising questions about chance,' the team explained.
To get to the bottom of it, the team analysed data from 58,007 US women who had had at least two children.
The researchers investigated the impact of eight maternal traits - height, body mass index, race, hair colour, blood type, chronotype (whether or not you're an early bird or a night owl), age at first menstruation, and the age when they had their first child.
What determines the sex of a baby?
The sex of a baby is determined by the combination of sex chromosomes inherited from the sperm and egg.
While the egg always carries an X chromosome, the sperm can either carry an X or a Y.
If the sperm carrying an X chromosome fertilises the egg, the embryo will be female, while the embryo will be male if it's a Y chromosome.
Seven of these traits did not impact the sex of the baby.
However, their analysis uncovered a key link between the age at which the mother had their first child, and whether or not they had boys or girls.
Women who were older than 28 when they first gave birth had a 43 per cent chance of having children of only one sex.
But women who were younger than 23 when they first became a mother only had a 34 per cent chance of having children of only one sex.
'Older maternal age may be associated with higher odds of having single-sex offspring, but other heritable, demographic, and/or reproductive factors were unrelated to offspring sex,' the researchers explained.
The reason for this link remains unclear.
However, the researchers say it could be associated with the physiological changes women go through as they age - including a shorter follicular phase, and a lower vaginal pH.
'A shorter follicular phase tends to favor Y chromosome survival, whereas a more acidic vaginal environment favors X chromosome survival,' the team explained.
'Each woman may have a different predisposition to each of these factors as they age, which could lead to a higher probability of consistently producing same-sex offspring.
'However, these mechanisms remain speculative, and more detailed data are required to confirm these hypotheses.'
Overall, the findings suggest that the chance of having a boy or girl is not 50/50 after all.
'Families desiring offspring of more than one sex who have already had two or three children of the same sex should be aware that when trying for their next one, they are probably doing a coin toss with a two-headed coin,' they team concluded.
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