Why Do Some Women Only Give Birth to Boys or Girls? A Genetic Explanation

Updated: February 12, 2020
Why do some women only give birth to boy or girl babies? It could be, at least in part, due to genetics. Doctors weigh in with what can genetically impact the sex of a baby.
woman holding sonogram to determine sex of baby

I am one of three sisters; my mom has three sisters, and my dad has three sisters. My husband and I have two girls. Most of my first cousins are female—in fact, it’s a long-running joke that women dominate both sides of my family and that if you’re a male in my family (sorry, dad and bro!) you’ve got the patience of saints!

I’ve heard for years—and my own OBGYN swore to me—that when it comes to gender in utero, it’s the male that determines whether baby is born female or male. Ok, fine. The fastest “sperm” determines gender. Got it.

More: 8 Traits Babies Inherit From Their Father

But how come when I meet a family with only daughters (such as my mom and her three sisters; no brothers) or a family that’s chock-full of only sons, I wonder: is gender in utero really just 50/50, or do some women out there really give birth to only boys or only girls?

Curious, I asked the experts and this is what I found out…

Which Chromosome Determines the Sex of the Baby?

According to Dr. Joel Gator Warsh, a Southern California-based integrative pediatrician, we do not know why some women tend to have only boys or girls. “The sperm determines the sex of a baby depending on whether they are carrying an X or Y chromosome. An X  and Y combine to make a boy, while an XX make a girl.”

Ok, we got that part down to a science, literally. But a woman giving birth to five boys or five girls (only) cannot just be a coincidence...can it?

The Math Behind Determining a Baby's Sex

Dr. Kimberly Langdon, an Ohio-based OBGYN and medical advisor at Medzino, a digital health company based in California and Germany, tossed some (simplified!) math and science our way to better understand gender assignment.

“What we have been taught conventionally is that—bar any genetic disorders that cause early pregnancy loss that only affect girls or boys—there is always a 50/50 chance of one or the other gender each time. The chance of a girl after 3 boys is still the same probability.”

However, she adds, men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. However, in women, it is unpredictable. (More research is needed...hint, hint scientists!)

Dr. Langdon references a Newcastle University study published in Evolutionary Biography that suggests that an as-yet undiscovered gene controls whether a man’s sperm contains more X or more Y chromosomes, which affects the sex of his children.

How Sperm Influences the Sex of a Baby

A gene, explains Dr. Langdon, consists of two parts, known as “alleles,” one inherited from each parent. “Newcastle researcher Corry Gellatly demonstrates that it is likely men carry two different types of allele, which results in three possible combinations in a gene that controls the ratio of X and Y sperm.”

Therefore, adds Dr. Langdon, men with the first combination, known as mm, produce more Y sperm and have more sons. “The second, known as mf, produce a roughly equal number of X and Y sperm and have an approximately equal number of sons and daughters. The third, known as ff produce more X sperm and have more daughters.”

What's the Bottom Line?

According to Dr. Langdon, “The gene that is passed on from both parents, which causes some men to have more sons and some to have more daughters, may explain why we see the number of men and women roughly balanced in a population.”

Adds Dr. Warsh: “Why some women have more males than females remains unknown. What is known is that that percentage is shrinking. A study from 1998, published by JAMA, showed the percentage of males to females is shrinking. Since 1950, significant declines in the proportion of males born have been reported in Europe, Canada and the USA.  The rates in this study showed a ratio of males to female births of 0.515 in 1950 to 0.513 in 1994.”

According to ScienceFocus, “In a sense nearly all women are predisposed to have more boys – the average sex ratio of 105 boys to 100 girls is influenced by partner choice, which will have a genetic component. So we might expect genetic effects in women too, albeit weak ones.”

So, more research is needed to give us a real definite answer about why some women tend to birth the same gender—we’ve got some studies, but not much that is concrete.

As Dr. Alyse Kelly-Jones, an OB-GYN based in Charlotte, North Carolina, assured me, like my own doctor did: “The bottom line is birth gender is a flip of the coin. You have a 50/50 shot each time of getting a boy or a girl.”

But as the fun little song lyric turned chat says, “Who rules the world? Girls!

Yup, girls—some early research studies confirmed so. Just kidding. Maybe!

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