Why Do I Keep Having Girls ... (Or Boys)? The Genetics Behind Gender

Updated: October 27, 2021
Our experts have studied the science and put together this clear explanation as to why some women keep having babies of the same sex.

Why do some women only give birth to boy or girl babies? It could be, at least in part, due to genetics. Doctors have weighed in on what can genetically impact the sex of a baby.

We all know that the X chromosome and Y chromosome determine whether a woman is going to have a baby girl or a baby boy. But, why are some women having all boys while others are having all girls? Is it a case of family history? Is there something that gives pregnant women an increased chance of giving birth to a baby with the same sex chromosomes over and over again? We're going to take a look at why some women ask, “Why do I keep having girl babies?” while others wonder why they keep having boys.

More: 8 Traits Babies Inherit From Their Father

How Sex Chromosomes Determine Whether You're Having a Baby Girl or Baby Boy

Some pregnant women may believe that environmental factors may play a role in what sex baby they have, while others look at their family trees. Whether you're having a baby for the first time or the fourth, we all really know that it's a matter of science.

The cells in our bodies are made up of 46 chromosomes that are grouped by pairs. Each pair has one chromosome from the mother and one from the father. Each sex cell carries half of the chromosomes. When it comes to the mother's eggs, chromosome 23 is always X. For the father's sperm, chromosome 23 can be either X or Y.

“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," says Dr. Joel Gator Warsh, a Southern California-based integrative pediatrician.

Although it is the man's sperm that ultimately determines the sex of a baby, is it a coincidence that some women give birth to only boys while others only have baby girls? Math and science have a lot to say about it!

The Math Behind Determining a Baby's Sex

One study suggests that looking at family trees could give some insight into why some couples keep having only girls or only boys. A Newcastle University study looked at the family history of nearly 1,000 couples to try to get down to the bottom of why some families are all girls or all boys. Researchers found that men are more likely to have sons if they have more brothers and are more likely to have daughters if they have sisters. But, in women, the likelihood of having a girl or boy just couldn't be predicted.

While some researchers look at family trees, others look at some simple math when it comes to sex chromosomes and the number of boys or number of girls in a family.

Dr. Kimberly Langdon, an Ohio-based OB-GYN 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 three boys is still the same probability.”

However, Langdon also references the Newcastle University study, published in Evolutionary Biography that showed that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. It 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. Looking at how sperm cells impact the sex of the baby is key to unraveling why some women have only boys and others have only girls.

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.”

As we can tell from a variety of studies, more research needs to be done into why women only give birth to boys or girls. Much of the research has been focused on men because they have the deciding chromosome, but more studies can be done to look at womens' family trees to see if any type of pattern can be determined.

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 that 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, produces a roughly equal number of X and Y sperm, and [men with an mf combination] have an approximately equal number of sons and daughters. The third combination, known as ff, [causes men to] produce more X sperm and have more daughters.”

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.”

But, one study shows that girls may be taking over.

Dr. Warsh tells Family Education, " 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.”

The Bottom Line

Although there have been studies to look at why some pregnant women give birth to all boys or all girls, the topic is still one that baffles many in the medical field.

More research is needed to give us a real definite answer about why some women tend to birth the same gender.

“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, " says Dr. Alyse Kelly-Jones, an OB-GYN based in Charlotte, North Carolina.

In the end, whether you have all boys or girls, any healthcare provider will tell you that having a healthy baby of either gender is what is most important.