What determines the sex of a baby?

Dr. Jeanne Morrison, PhD
Family Practitioner

In humans, the sex of the baby is determined by the sperm. The sperm gamete is heterogamatic because approximately half of them contain the X chromosome which will result in a girl and approximately half of them contain the Y chromosome which will result in a boy. On the other hand the ova gamete is homogametic, in other words all its cells possess the X chromosome. Thus the ova gamete does not influence the baby’s sex. Therefore when a sperm penetrates the ova and fertilization occurs there are two possible outcomes: XX or a girl and XY or a boy.

Sperm dictates the sex of the fertilized egg. However, a lot of mutual factors contribute to gender, including as the egg's receptivity to the male's chromosomes. So if a couple has five girls, that doesn't necessarily mean it's the man's fault. You can blame a man for keeping his eyes glued to the TV during a conversation, but you can't pin this one on him. Both partners have some role in determining the sex of the offspring.

As many people know (or have heard), the sperm that carry male chromosomes (which will make a boy) can swim faster because they carry less genetic material and are lighter. So though male and female sperm have equal muscular power in their flippers, the lighter ones will travel faster (just like a smaller cyclist will go faster than a larger one, if all other things are equal, because there's less mass to move).

But even if the male sperm outrace the female sperm, the male sperm will die off if the egg isn't in proper position for fertilization, leaving the female sperm clear for landing. Although male sperm are faster, female sperm are like tortoises; they keep pushing ahead slowly, hanging out, and waiting a few days. To have a boy, having sex around ovulation is critical, or the female sperm will win.

Aside from timing it right (and praying), you can do little to influence gender during conception. Some couples may try copulating upside down or in other acrobatic positions because they've heard rumors that you can influence the gender of a child, but the process is more complex than that.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.