Rhianna Benson
For decades now, biology teachers have taught school children that the chances of a baby being born a boy or a girl are bang on 50-50.
The theory has long professed that the chromosome inherited from the father's sperm is what determines whether or not a baby becomes a male or a female. Their chromosome blends with the mother's, which is always X.
So, as per this explanation, a dad's Y chromosome - when merged with the mum's during fertilisation, which occurs during intercourse - would see the baby's cells develop to become boy (being XY), and a dad's X chromosome would make them biological female (XX).
According to a new study conducted by Harvard University professors earlier this year, however, biological sex at birth isn't actually the even toss-up we think it is. In fact, some experts have gone as far as to say that its pretty predetermined. But how?
Advert
First things first, let's take a look at the nature of the investigation.
The intriguing new study was conducted several months ago by the members of the Harvard T.H. Chan School of Public Health, who published their findings in the journal Science Advances in July.
Over 146,000 pregnancies from more than 58,000 US participants between 1956 and 2015 were considered.
Amongst the numerous groundbreaking conclusions forged by scientists fronting the inquiry is one, as we say, that alleges that a baby's biological sex is far from random, and definitely not reliant on the father's genetics.
Instead, this factor is said to be more heavily swayed by the age of the mother during conception, as well as a number of other genes, and the sexes of the child's older siblings.
Offering an explanation for these findings, professor of nutrition and epidemiology, Jorge Chavarro - also the senior author of said study - told the Washington Post: "If you’ve had two girls or three girls and you’re trying for a boy, you should know your odds are not 50-50.
"You’re more likely than not to have another girl."
Considering these first two findings, it was discovered that women who began having children after turning 28 were much more likely to have all boys, or all girls. This, Chavarro said, could be due to the changes that occur in a woman's biology over time.
On top of this, two genes also emerged which were interpreted by scientists as being linked to having only boys, or only girls.
"We don’t know why these genes would be associated with sex at birth, but they are, and that opens up new questions," Chavarro continued.
And then looking to the latter, sibling-related explanation, it was similarly found that some families are simply more likely to have same-gendered offspring, with mothers of three or more kids either likely to welcome a set of male children, or a set of females.
Going forward, Chavarro believes it integral for future gender-based pregnancy studies consider further factors, including lifestyle, nutrition, and exposure to environmental chemicals to determine whether these also have a part to play.