Down syndrome produces a recognizable pattern of physical features, developmental differences, and associated health conditions that are usually identified at birth or shortly after. The symptoms range widely from person to person. Some individuals have mild physical features and near-typical development, while others experience significant health challenges requiring ongoing medical care.
Physical Features Present at Birth
Most of the visible signs of Down syndrome are apparent from the moment a baby is born, though they become more noticeable as the child grows. The most common facial features include a flattened face (particularly across the bridge of the nose), almond-shaped eyes that slant upward, and a tongue that tends to stick out of the mouth.
Beyond the face, other physical signs include a short neck, small ears, small hands and feet, and shorter-than-average height. Many babies have a single crease running across the palm of each hand rather than the typical two lines. Small, slightly curved pinky fingers are also common. None of these features on their own indicates Down syndrome, but together they form a recognizable pattern that prompts genetic testing to confirm the diagnosis.
Low Muscle Tone in Infancy
One of the earliest and most functionally important symptoms is hypotonia, or low muscle tone. Babies with Down syndrome feel noticeably “floppy” when held. Their arms and legs offer less resistance to movement than expected, and they have difficulty sustaining posture and moving against gravity. This isn’t muscle weakness in the traditional sense. It’s a difference in how the brain signals the muscles to maintain tension.
Low muscle tone affects nearly every early milestone. Babies with Down syndrome typically take longer to hold up their heads, sit independently, crawl, and walk. Feeding can also be more difficult in the first months of life because the muscles of the mouth and tongue are affected too. With physical therapy and consistent practice, most children do reach these motor milestones, just on a delayed timeline.
Cognitive and Learning Differences
Down syndrome causes some degree of intellectual disability in all cases, but the range is wide. Most individuals fall in the moderate range, with IQ scores between 35 and 50, though some are in the mild range (50 to 70) and a smaller number have more significant impairment. These differences emerge early in childhood and affect areas like memory, language processing, and the ability to pick up on patterns.
Cognitive ability in Down syndrome is not fixed or entirely predictable at birth. Early intervention, quality education, and an enriched environment all influence outcomes. Many adults with Down syndrome hold jobs, maintain friendships, and live with varying degrees of independence.
Speech and Language Delays
Language development follows a distinctive pattern. Children with Down syndrome tend to produce their first words around 21 months, which roughly aligns with their cognitive development but lags behind the typical 12-month mark. Receptive language, the ability to understand what others say, tends to be stronger than expressive language, the ability to speak and form sentences. This gap can be a source of real frustration for young children who understand far more than they can communicate.
Several factors compound the delay. Children with Down syndrome are less likely to pair their gestures with vocalizations during the babbling stage, and they often struggle with word segmentation, the skill of picking individual words out of a stream of speech. Reduced speech intelligibility is common throughout childhood. Sentence structure tends to be more delayed than vocabulary, meaning a child may know many words but have trouble stringing them together grammatically. Speech therapy starting in infancy can make a significant difference.
Heart Defects
Between 50 and 65 percent of babies born with Down syndrome also have a congenital heart defect. This is the most common serious medical complication, and it’s the reason every newborn with Down syndrome receives a cardiac evaluation. The types of heart defects vary widely, from holes between the heart’s chambers to problems with how the valves open and close. Some defects are mild and resolve on their own. Others require surgical repair in the first year of life. Advances in pediatric heart surgery have dramatically improved survival and long-term outcomes for these children over the past several decades.
Hearing and Vision Problems
Sensory issues are extremely common. Up to 75 percent of children with Down syndrome experience some degree of hearing loss, which can range from mild fluid buildup in the middle ear to more permanent forms. Because hearing directly affects language development, even mild losses can compound existing speech delays if they go undetected. Regular hearing screenings are a standard part of care.
Vision problems affect a similarly high proportion. Up to 60 percent develop eye conditions like cataracts, and about half need glasses for refractive errors such as nearsightedness or farsightedness. These issues can appear in infancy or develop later in childhood, so routine eye exams are important throughout life.
Thyroid Dysfunction
The thyroid gland, which controls metabolism and energy levels, is frequently underactive in people with Down syndrome. Congenital hypothyroidism occurs at roughly 28 times the rate seen in the general population. Beyond the newborn period, the incidence of abnormal thyroid levels rises further, with some studies finding elevated markers in up to 85 percent of infants under 12 months. The lifetime prevalence of thyroid disease in Down syndrome ranges from 13 to 63 percent.
Because thyroid problems develop gradually and their symptoms (fatigue, weight gain, sluggishness) overlap with features of Down syndrome itself, they’re easy to miss without blood testing. Current guidelines recommend thyroid screening at 6 months, 12 months, and then annually throughout life.
Digestive Issues
Gastrointestinal problems are common at every age. Over three-quarters of newborns with Down syndrome seen in clinics have some form of gastrointestinal issue, ranging from feeding difficulties to structural abnormalities that require surgery. About 6 percent are born with intestinal blockages such as duodenal atresia, where part of the small intestine is closed off. Imperforate anus, a condition where the anal opening hasn’t formed properly, also occurs at higher rates.
Hirschsprung disease, which affects about 2 percent of children with Down syndrome, causes a section of the large intestine to lack the nerve cells needed to push stool through. It shows up as a distended belly, poor weight gain, vomiting, and severe constipation. Less dramatic but still persistent digestive symptoms like acid reflux, constipation, diarrhea, and abdominal discomfort are common in both children and adults. Celiac disease, an autoimmune reaction to gluten, is also associated with Down syndrome and can appear at any age.
Growth Patterns
Children with Down syndrome are shorter than their peers, with proportionally shorter limbs and smaller head circumferences. Weight tends to be closer to typical ranges in early childhood, but body mass index runs higher than average from age two onward. This reflects a combination of shorter stature, lower muscle mass, and a tendency toward weight gain that becomes more pronounced with age. Doctors use Down syndrome-specific growth charts rather than standard pediatric charts to track development accurately and distinguish normal variation from a genuine health concern.
Sleep Apnea
Obstructive sleep apnea is very common in Down syndrome, driven by a combination of low muscle tone in the airway, a smaller midface, and, in many children, enlarged tonsils and adenoids. During sleep, symptoms include snoring, pauses in breathing, restless tossing, gasping or choking, and mouth breathing. Some infants don’t snore at all but simply sleep poorly.
The daytime effects are easy to misattribute to other causes. Children with untreated sleep apnea often have behavior problems like hyperactivity, impulsiveness, or aggression. They may have morning headaches, difficulty concentrating, or poor performance in school. Because these symptoms overlap with the cognitive features of Down syndrome, sleep apnea can go unrecognized for years. A sleep study is the standard way to diagnose it, and treatment can meaningfully improve behavior, learning, and quality of life.
How Symptoms Vary by Genetic Type
About 95 percent of Down syndrome cases are caused by a full extra copy of chromosome 21 in every cell. A smaller percentage result from translocation (where part of chromosome 21 attaches to another chromosome) or mosaicism (where only some cells carry the extra chromosome). In mosaic Down syndrome, the presence of cells with the typical 46 chromosomes can result in milder features, though this isn’t guaranteed. Some individuals with mosaicism have most of the classic characteristics, while others have very few. The percentage of affected cells doesn’t reliably predict how someone will be affected, so mosaicism doesn’t come with a clear prognosis at diagnosis.