Down syndrome is slightly more common in males. Across large studies, roughly 56% of babies born with Down syndrome are male and 44% are female, producing a male-to-female ratio of about 1.3 to 1. For comparison, the general population ratio at birth is 1.06 males for every female. So while both sexes are affected, boys with Down syndrome consistently outnumber girls.
How Large Is the Difference?
The male surplus is modest but consistent across populations worldwide. A meta-analysis of cytogenetic data from multiple countries found a sex ratio of approximately 1.3 males per female among people with standard trisomy 21, the form responsible for about 91% of all Down syndrome cases. A separate large study of over 1,100 confirmed diagnoses found 612 males and 517 females, a ratio of 1.18 to 1. The exact number varies between studies, but the direction is always the same: more males.
This skew applies specifically to “free” trisomy 21, where a full extra copy of chromosome 21 is present in every cell. In rarer forms of Down syndrome caused by chromosomal translocations or mosaicism (where only some cells carry the extra chromosome), the male-to-female gap is less pronounced.
Why More Males Are Born With It
Researchers don’t have a single confirmed explanation, but the evidence points to differences in fetal survival rather than differences in how often the extra chromosome occurs in the first place. Among early miscarriages involving trisomy 21, studies have found more females than males. This suggests that female embryos with the condition may be slightly more likely to be lost early in pregnancy, leaving a higher proportion of surviving male pregnancies that continue to birth.
This pattern is somewhat counterintuitive. In the general population, male embryos are already conceived more often than female embryos, but they’re also more vulnerable to miscarriage, which narrows the gap by the time babies are born. With trisomy 21, the usual survival advantage that female embryos have seems to be reduced or reversed, and the result is an even larger male surplus at birth than you’d see otherwise.
Maternal Age Changes the Ratio
One of the more striking findings is that the male-to-female gap in Down syndrome is not constant across all mothers. It shifts dramatically with maternal age. Among younger mothers (ages 20 to 25), the sex ratio in Down syndrome births climbs as high as 1.84 males per female, nearly twice as many boys as girls. As maternal age increases, the ratio steadily drops. By age 35 and older, the sex ratio among babies with Down syndrome is close to that of the general population, around 1.06.
This age-related pattern suggests that whatever biological mechanism drives the male excess is most active when younger women conceive a child with trisomy 21. The reasons remain unclear, but it may relate to differences in how the uterine environment responds to chromosomally abnormal embryos at different ages.
Health Differences Between Males and Females
Beyond the numbers at birth, there are meaningful differences in how Down syndrome affects males and females later in life, particularly around fertility. Between 35% and 50% of women with trisomy 21 are fertile, and those who become pregnant have up to a 50% chance of having a child who also has Down syndrome. Men with Down syndrome, on the other hand, are typically infertile unless they have the mosaic form, where not all cells carry the extra chromosome.
Both sexes share the same core health profile associated with Down syndrome, including higher rates of congenital heart defects, thyroid problems, and immune system differences. The frequency of these conditions does not appear to differ substantially between males and females with the diagnosis.
Overall Prevalence
Regardless of sex, Down syndrome occurs in about 1 in every 640 births in the United States, with roughly 5,700 babies born with the condition each year. Globally, the United Nations estimates an incidence of 1 in 1,000 to 1 in 1,100 live births, though this number varies by country depending on maternal age demographics and access to prenatal screening.
The male predominance, while real and statistically significant, is not dramatic enough to make Down syndrome a “male condition” the way some X-linked genetic disorders are. It affects both sexes across all populations, with boys holding a slight but consistent edge in frequency.