People with Down syndrome have 47 chromosomes instead of the usual 46. The extra chromosome is a third copy of chromosome 21, which is why the condition is also called trisomy 21. This is the most common form, but not the only one. About 5,700 babies are born with Down syndrome each year in the United States, roughly 1 in every 640 births.
Why There Are 47 Instead of 46
Human cells normally contain 23 pairs of chromosomes, totaling 46. Each parent contributes one chromosome to every pair. Down syndrome occurs when a child ends up with three copies of chromosome 21 rather than two, bringing the total to 47.
This extra copy arises from an error during cell division called nondisjunction. When egg or sperm cells form, paired chromosomes are supposed to separate so each resulting cell gets one copy. Sometimes chromosome 21 fails to separate properly, producing a reproductive cell with two copies instead of one. When that cell combines with a normal cell from the other parent, the embryo has three copies total.
The vast majority of these errors happen in the egg rather than the sperm, and most occur during the first stage of the egg cell’s division. Egg cells begin developing while a female fetus is still in the womb, then pause for years or decades before completing division at ovulation. That long pause appears to increase the chance of chromosomes separating incorrectly, which helps explain why the risk rises with maternal age.
Three Types With Different Chromosome Arrangements
Not every case of Down syndrome looks the same at the chromosomal level. There are three distinct types, and they differ in how the extra chromosome 21 material is distributed.
Trisomy 21 is the most common form, accounting for about 95% of cases. Every cell in the body carries 47 chromosomes, with a full extra copy of chromosome 21.
Translocation Down syndrome makes up about 3–4% of cases. Here, the total chromosome count can be either 46 or 47, because the extra chromosome 21 material is physically attached to another chromosome (often chromosome 14). The person still has three copies’ worth of chromosome 21 genetic material, but it may not show up as a separate, standalone chromosome. This type is notable because in about one-third of cases, a parent carries the translocation in a balanced form without symptoms, which means it can run in families.
Mosaic Down syndrome is the rarest form, occurring in about 1–2% of cases. People with mosaicism have two different cell populations: some cells carry the typical 46 chromosomes, while others carry 47. The ratio varies from person to person and even from tissue to tissue within the same person. The proportion of cells with the extra chromosome influences how many features of Down syndrome a person experiences, so symptoms can range from very mild to similar to full trisomy 21.
How Maternal Age Affects Risk
A woman’s age at conception is the strongest known risk factor. A large Danish study of more than 500,000 pregnancies put concrete numbers on the relationship. Among women aged 20 to 29, trisomy 21 occurred in about 0.10% of pregnancies. For women 35 to 39, that rose to 0.62%. By ages 40 to 44, the rate reached 2.14%, and for women 45 and older, 3.22%.
Put another way, compared to women in their twenties, those between 40 and 44 had roughly a 16-fold increased risk, and women 45 and older had a 36-fold increased risk. The reason traces back to those egg cells sitting in a paused state of division for decades. The longer the pause, the more likely the chromosome separation machinery is to malfunction.
That said, most babies with Down syndrome are born to women under 35 simply because younger women have far more pregnancies overall.
How the Extra Chromosome Is Detected
Prenatal screening can identify trisomy 21 with high accuracy well before birth. A blood test called non-invasive prenatal testing (NIPT) analyzes fragments of fetal DNA circulating in the mother’s blood as early as 10 weeks of pregnancy. For Down syndrome specifically, NIPT achieves over 99% sensitivity and over 99% specificity, meaning it catches nearly all cases and produces very few false alarms. The positive predictive value, which reflects the chance that a positive result is truly correct, ranges from 85% to 99% depending on the mother’s baseline risk.
Because NIPT is a screening test rather than a diagnostic one, a positive result is typically confirmed with amniocentesis or chorionic villus sampling. These procedures collect fetal cells directly so the chromosomes can be counted and examined under a microscope, producing a definitive answer. After birth, the same chromosome analysis (called a karyotype) confirms the diagnosis and identifies whether the type is standard trisomy, translocation, or mosaic.
What the Extra Chromosome Actually Does
Chromosome 21 is the smallest human chromosome, but it still contains several hundred genes. Having three copies means cells produce roughly 50% more of every protein coded by those genes. That overproduction disrupts the delicate balance of development throughout the body, particularly in the brain and heart.
The effects are wide-ranging. Intellectual disability is universal, though its severity varies considerably. About half of babies with Down syndrome are born with a heart defect. Low muscle tone, a flattened facial profile, and shorter stature are common physical features. The immune system functions differently, leading to higher rates of certain infections. Thyroid disorders, hearing problems, and vision issues are also more frequent.
Life expectancy has improved dramatically over the past several decades, largely due to advances in heart surgery and better medical care. Most people with Down syndrome now live into their 60s, and many live active, semi-independent lives with appropriate support.