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Boy or Girl: Who Determines the Sex of a Baby?

Boy or Girl: Who Determines the Sex of a Baby?

08:37 3rd February 2017 | Sex Determination

Pregnant Sex of Baby Sex Chromosomes X Chromosomes Y Chromosomes Genes Boy Girl Sexual Organs Hormones Embryonic Gonad Male Structure Female Structure Ovaries Female Hormone First Trimester Ultrasound

Once a couple finds out they are pregnant, it is only natural for them to be curious if they will have a boy or a girl. This blog post will explain the biological process that determines the sex of the baby.

Basic Introduction on Sex Determination

Humans have one pair of sex chromosomes. Women have two “X” chromosomes, while men have an “X” and a “Y”. The presence or absence of the “Y” chromosome is what determines the sex of a baby.

Since women only have “X” chromosomes, all their eggs only have “X” chromosomes. However, because men have both an “X” and a “Y”, each of their sperm will carry one or the other. That means that the chromosome in the sperm that fertilises the eggs will determine the baby’s sex. So, ultimately the father determines the sex of the baby, even though he cannot control whether an “X” or “Y” reaches the egg first (unless a baby is conceived through IVF).


Humans have roughly 25 000 genes. Yet, only one gene located on the “Y” chromosome – the SRY gene – is needed for male development. How does one gene play such a significant role? The simplest way to explain this phenomenon is that this specific boy gene actually activates other boy genes. If this gene is not present, girl genes are activated.

Sexual Organs

Initially, the internal and external sexual organs of boys and girls develop from the same basic body plant. At birth, however, the sex is obvious.


At seven weeks, the embryonic gonad and two sets of ducts are influenced by male and female hormones to form male or female structure.

If there is a “Y” chromosome, the SRY gene will be activated. The production of the male hormone, testosterone, will begin and the sexual organs will begin to form. If there is no “Y” chromosome, the baby will be a girl. Ovaries will form and produce the female hormone, estrogen. This all starts by the end of the first trimester.

To determine the sex, an ultrasound can be done at 16 weeks, but it may not be 100% accurate. Generally an ultrasound between 18 – 20 weeks is more accurate, but each case is different.


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