Birth defects arise from a mix of genetic, environmental, and maternal health factors, but for roughly 80% of cases, no single cause can be identified. About 1 in every 33 babies born in the United States each year has a birth defect. Some defects are minor and need no treatment, while others affect the heart, brain, spine, or other organs in ways that require lifelong care. Understanding the known causes can help explain what happens during fetal development and, in some cases, reduce risk.
Genetic and Chromosomal Problems
The most straightforward causes of birth defects are errors in a baby’s DNA. These fall into two broad categories: chromosomal abnormalities and single-gene disorders.
Chromosomal abnormalities happen when a baby has too many or too few chromosomes, or when part of a chromosome is missing or rearranged. Down syndrome, caused by an extra copy of chromosome 21, is the most recognized example. These errors typically occur randomly during cell division and become more common as parental age increases.
Single-gene disorders result from a mutation in one specific gene passed down from one or both parents. Cystic fibrosis is a well-known example. A parent can carry the mutated gene without having the condition, which means the defect can appear unexpectedly in a family with no known history. Some single-gene conditions are sex-linked, meaning they primarily affect boys because the mutation sits on the X chromosome.
Infections During Pregnancy
Certain infections can cross the placenta and interfere with fetal development, particularly during the first trimester when organs are forming. Doctors group the most dangerous ones under the acronym TORCH: toxoplasmosis, syphilis, rubella, cytomegalovirus (CMV), and herpes simplex.
Rubella is one of the most damaging. A baby exposed in utero can develop congenital rubella syndrome, which includes cataracts, heart defects (especially holes between the heart’s chambers), and hearing loss. Widespread vaccination has made rubella rare in the U.S., but it remains a threat in countries with lower vaccination rates.
CMV is actually the most common congenital infection. It can cause growth restriction, hearing loss, vision problems, and abnormal calcium deposits in the brain tissue surrounding the ventricles. Many infected newborns appear healthy at birth but develop hearing loss or learning difficulties months or years later.
Toxoplasmosis, spread through undercooked meat or contact with infected cat feces, can lead to growth restriction, eye damage, and brain calcifications. Syphilis, if untreated during pregnancy, can cause bone abnormalities, hearing loss, and distinctive dental malformations. Herpes simplex rarely infects the fetus before birth but can cause severe illness in newborns exposed during delivery, including brain inflammation and widespread organ damage.
Maternal Diabetes and Obesity
A mother’s metabolic health before and during pregnancy has a direct effect on how the baby’s organs develop, particularly the heart. In large population studies, pre-existing diabetes is one of the strongest known risk factors for congenital heart defects. Higher blood sugar levels in the first trimester, measured by hemoglobin A1C, correlate with greater risk. Women with more diabetes-related complications are also more likely to have a baby with a heart defect.
Obesity increases risk in a similar pattern. A Swedish study of over 2 million births found that the risk for specific heart defects rose progressively with BMI. Mothers with a BMI above 25 had higher rates of certain defects like holes between the heart’s upper chambers, while those with a BMI above 35 faced elevated risk for more complex structural problems. The biological explanation involves several overlapping mechanisms: obesity creates chronic inflammation, raises insulin levels, reduces blood vessel function, and alters the hormonal signals that regulate placental nutrient transfer. All of these can disrupt the precisely timed process of organ formation in the embryo.
Alcohol and Substance Exposure
Alcohol is one of the most well-documented causes of preventable birth defects. When a pregnant person drinks, alcohol crosses the placenta freely and can damage the developing brain and face through several pathways.
The characteristic facial features of fetal alcohol syndrome, including a smooth ridge between the nose and upper lip, thin upper lip, and small eye openings, trace back to a specific group of embryonic cells called the cranial neural crest. These cells normally migrate into position to form facial cartilage and bone. Alcohol exposure during this early migration window triggers excessive cell death among these cells, leading to the distinctive underdevelopment.
Brain damage from alcohol is broader and more complex. Alcohol generates harmful molecules called reactive oxygen species that damage cell structures, particularly the mitochondria that power each cell. When mitochondria break down, they release signals that trigger cell death. Alcohol also blocks key growth factors that tell brain cells when to divide and survive, and it disrupts the scaffolding cells that guide developing neurons to their correct positions in the brain. Neurons that lose this guidance end up in the wrong layers of the cortex, which can cause lasting problems with learning, behavior, and coordination.
Smoking, certain prescription medications, and recreational drugs can also cause birth defects, though the mechanisms and risks vary. The first trimester poses the highest risk for most substances because that is when the major organ systems are forming.
Paternal Age
The father’s age also plays a role, though it receives less attention than maternal factors. A systematic review and meta-analysis found that fathers aged 40 and older had higher rates of offspring with cardiovascular abnormalities (10% increased risk), urogenital abnormalities (28% increased risk), facial deformities, and chromosomal disorders (30% increased risk) compared to fathers aged 25 to 29. Interestingly, very young fathers under 20 also showed elevated risk for urogenital and chromosomal problems. The likely explanation is that sperm accumulate genetic mutations over time, and very young fathers may have less mature reproductive biology.
Folic Acid and Nutritional Deficiencies
One of the clearest success stories in birth defect prevention involves folic acid, a B vitamin essential for the early formation of the brain and spinal cord. The CDC recommends that all women capable of becoming pregnant take 400 micrograms of folic acid daily. This single step helps prevent neural tube defects like spina bifida and anencephaly, conditions where the spinal cord or brain doesn’t close properly during the first few weeks of pregnancy.
The timing matters: the neural tube closes by about 28 days after conception, often before a woman knows she’s pregnant. That’s why the recommendation applies to all women of reproductive age, not just those actively trying to conceive. Folic acid is available in most daily multivitamins and is also added to enriched grain products like bread and cereal in the U.S.
Most Cases Have No Clear Cause
Despite the known risk factors listed above, the reality is that most birth defects can’t be traced to a single identifiable cause. A population-based study that systematically reviewed birth defect cases over five years found that a specific cause could only be assigned in about 1 in 5 cases. The remaining 79.8% were classified as unknown. Many of these likely result from complex interactions between multiple genes and environmental exposures, combinations that are difficult to untangle with current science.
This means that parents of a child born with a defect often receive no clear explanation for what happened. It also means that while steps like taking folic acid, managing blood sugar, avoiding alcohol, and staying up to date on vaccinations genuinely reduce risk, they can’t eliminate it entirely.
When Birth Defects Are Detected
Prenatal screening can identify many birth defects before delivery. First-trimester screening between 11 and 13 weeks uses a combination of blood tests and ultrasound to check for certain heart defects and chromosomal disorders. Second-trimester screening between 15 and 20 weeks includes blood tests and a comprehensive ultrasound, typically performed around 18 to 20 weeks, that examines all major structures of the baby’s body. A fetal echocardiogram provides detailed images of the heart specifically. Some defects, particularly those involving internal organs or brain development, may not become apparent until after birth or even later in childhood.