Boy or Girl: How Genetics Determine Your Baby’s Gender

By January 3, 2019General

It’s a question that many mothers ask as soon as they learn they are pregnant. Will it be a boy or girl? While the answer to this will be revealed in months to come, what factors actually determine your baby’s gender?
Boy Girl Baby

It’s one of the most exciting parts of birthing a new child. What will it’s gender be? While you wait for that exciting moment, here’s a bit about how genetics determine your baby’s gender.

X and Y Chromosomes

The genetic material that determines if a baby will be male or female is located within the X and Y chromosomes. Males have an X and a Y chromosome, while females have two X chromosomes.

Here’s where it gets interesting, male and female gametes i.e sperm or egg cells, have a single half of this genetic makeup. Eggs can only carry an X chromosome, while sperm cells can carry either X or Y. What this means is that the sex of the baby depends on which kind of sperm gets to the egg first.

What then?

The testis-determining factor, otherwise known as sex-determining region Y protein (SRY), is the DNA-binding protein responsible for initiating male sex growth in humans.

During week 7 of pregnancy, if the SRY gene is present, it begins to express itself in the fetus. The baby, which is the size of a blueberry, will begin to produce testosterone and grow male genitalia. If the gene is not present at this stage, then the baby will grow into a female.

What Are the Chances?

About half of a father’s sperm will create a boy and the other half will create a girl, so generally, there is a 50 – 50 chance parents will produce a boy or a girl.

However, it is not always that simple and across the world, slightly more boys (51%) are born than girls each year. One explanation for this suggests that as the male (Y) chromosome is slightly smaller than the female (X), they are slightly more lightweight and slightly quicker, meaning the sperm are more likely to make it to the egg first.

The lives of parents can also have a determining effect on the sex of the baby. Numerous studies have shown that various factors from location, nutrition and even wealth can affect the chances of having a boy or a girl. Despite this, it is important to consider that none of these effects are very considerable and it would be pointless and ineffective to try and influence the odds.

Various techniques have been tried to preselect the sex of a baby. These include various dietary supplements, acid/bicarbonate douches, sex at different times in a woman’s cycle relative to ovulation, dietary modifications, even having a woman stand on her head after intercourse. Predictably, either individually or in combination (the Shettles method for example) none of them work.

Using IVF with pre-implantation selection of the sex of the embryo is effective, but is illegal in this country, unless the couple carry a sex linked lethal condition such as Haemophilia or Duchenne Muscular Dystrophy.

Still, everyone knows a family where all children are of the same gender, but the odds are the same as flipping a coin. If you flip a coin 5 times and get heads, what are the chances of getting heads the next time? The answer is still 50%, as the coin has no memory of what results have passed and what will come. The same principle applies to the mother’s body. The only major factor is which type of sperm reaches the egg first, so for instance, in a family with all boys, it turned out to be a sperm carrying a Y chromosome each time.

The best advice, is that whether your baby is a boy or a girl, it is equally deserving of love, care and every opportunity you can offer it.

If you have any questions about your pregnancy or any other questions about your baby, please don’t hesitate to get in touch.
Dr Bevan Brown is one of the most trusted obstetricians in Sydney. Personalised care and strong relationships with our patients is of utmost importance!