The Bottom Line

It is estimated that birth defects affect an average of 3% of all pregnancies, which includes both structural and functional abnormalities; approximately 7,000 different types have been identified.

Birth defects have a wide range of effects on an individual’s quality of life, but can, in many cases, have very little overall impact.

However, some birth defects can be serious and even fatal, therefore various screening tests are available during pregnancy to attempt to potentially identify and prepare for possible complications.

Most birth defects do not have a known or confirmed cause, but to date, most are believed to occur from a combination of chromosomal/genetic and environmental factors.

Although research into these environmental factors is new and considered very incomplete, these agents (medications, chemicals, viruses) are the only type of causal factors that may be actively preventable.

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There is no universally accepted definition of what constitutes a birth defect. The term used to be specific to physical or structural problems of the body, but modern definitions also include abnormalities in function, metabolism, or body chemistry, that occur from conditions before conception (i.e. genetics) to during pregnancy (i.e. environmental).

Birth defects can affect almost any part of the body and can range from mild to severe, to treatable to possibly fatal. However, most children can lead completely healthy lives, with a normal lifespan.

Photo by Daniel Reche from Pexels

Birth defects can be found before birth, at birth, or any time after birth, but most are found within the first year of life; others can remain hidden for years. Some types of birth defects include:

  • Nervous system or brain problems

  • Structural (heart and neural tube – the most common)

  • Functional or developmental

  • Sensory problems (hearing, sight)

  • Metabolic disorders

  • Degenerative disorders (i.e. muscular dystrophy)

Approximately 2% to 5% of pregnancies are complicated by birth defects, of which more than 7,000 different types have been identified.  The most common figure reported is 3% of all pregnancies, which is considered the “expected background rate”, and is used as comparison in research studies.

For example, in general, when a study evaluates the potential cause (i.e. medication) of any specific birth defect, researchers look to determine whether the amount of birth defects in that exposed group was higher or lower than the expected background rate of 3%. 

If the rate is similar to 3%, the study will indicate that within that studied group, it cannot be determined if the medication caused any of the identified defects, as it is the same as would have been expected in an unexposed group (i.e. “due to chance”) – as long as the studied group was large enough.


The first trimester of pregnancy (or the first three months) is often indicated as the most vulnerable period for the development of birth defects, especially physical/structural. However, this assessment is outdated based on what is currently known about fetal development and teratology.

Although most organs form prior to 12 weeks of pregnancy, birth defects can occur during any stage of pregnancy, but likely do not start until after 4 weeks:

Environmental disturbances during the first 3 to 4 weeks of pregnancy (fertilization and implantation) are assessed to potentially cause miscarriage but not birth defects. This is because the first four weeks mainly involve the development of the amniotic sac, extraembryonic structures, and chorionic sac. Differentiation of specific cells and organs starts on the 29th day of gestation (approximately 4 weeks, 1 day).

Additionally, it is theorized the embryo may somehow correct itself from some harmful environmental exposure at this time. Therefore, the most vulnerable time period is likely weeks 4 through 16 (months 2 through 4).

However, some organs continue to develop (not just grow) throughout gestation. For example, the brain develops almost the entirety of pregnancy, especially mid-pregnancy, and the vulnerable period for fetal hearing is from 25 weeks' gestation to term (and 6 months into infancy).

Note: Even though the most vulnerable period is the first three months, this does not mean women need to wait longer than the first trimester to announce a pregnancy. This data is for birth defects specifically, not miscarriage. Further, if a fetus has cardiac activity after 8 weeks of pregnancy, and growth is as expected, the estimated miscarriage rate at that time is very low (about 1%).



Medical science has identified the cause of about 30% of birth defects. That means about 70% remain without a straightforward cause. Assessed categories of causes include genetics, chromosomal abnormalities, and/or environmental factors.

Chromosomal: A chromosome is made of a very long strand of DNA that contains genes. Every normal human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes.  Individuals who have either too many or too few chromosomes will receive a scrambled message regarding body development and function.

Genetic: A chromosome contains hundreds to thousands of genes. Since each gene on the chromosome controls the structure and function of a body part, individuals with defective genes will show defects in the corresponding body part to that gene.

Environmental: These types of disruptions are considered “external” factors, such as malnutrition, medications, drugs, chemicals, heavy metals, and viruses and bacteria (infections).

Birth defects are likely caused by more than one factor at the same time.

The majority of birth defects are considered the result of multiple environmental and/or genetic causes acting together.

Complex birth defects (60% to 70% of all birth defects) may involve a few interacting genes, many genes, the environment, or an interaction between genes and the environment. Additionally, some fetuses may have genetic risk factors that predispose them to certain conditions once an environmental factor acts or “triggers” it.

Of the known causes of birth defects, chromosomal or single gene abnormalities cause an estimated 25% to 30% of birth defects and are present in an estimated 0.5% of newborns.

Trisomy 21 (Down Syndrome, an extra copy of chromosome 21) is the most common chromosomal abnormality diagnosed in the United States every year.

"Trisomy" is the term used for any syndrome that occurs as the result of an extra chromosome.  Trisomy more commonly occurs as maternal age increases: for example, trisomy 21 occurs once in approximately 1,400 births among mothers aged 20 to 24, but then once in approximately 25 to 30 births in mothers older than 45.

Environmental causes are estimated to be responsible for about 5% to 10% of total known birth defects; however, this is the aspect of birth defects in which research is most incomplete.

Photo by Lisa Fotios from Pexels

Whether an exposure causes damage to an embryo/fetus depends on several factors, including the actual exposure, the stage of gestation, and the individual's genetic susceptibility (predisposition, mentioned above).

Until the 1940s and 50s, it was thought that embryos were protected from environmental agents such as drugs, viruses, and chemicals (see Thalidomide, under Limb Development). Therefore, research into environmental harms during pregnancy is still considered very new.

A literature review published in July 2021 assessed 66 systematic reviews and meta-analyses, determining that "congenital anomalies based on congenital heart diseases, cleft lip and palate, and others were associated with environmental risk factors based on maternal exposure to environmental exposures (air pollution, toxic chemicals), parental smoking, maternal history (infectious diseases during pregnancy, pre-gestational and gestational diabetes mellitus), maternal obesity, maternal drug intake, pregnancy through artificial reproductive technologies, and socioeconomic factors." 

In the United States, fetal exposure to alcohol is the most common environmental cause of babies born with birth defects and intellectual disability.

Incidence of cleft palate appears to be decreasing in the United States.

A study published in July 2021 reviewed a total of 13,808,795 pregnancies in the United States from 1999 to 2015. A total of 10,567 primary palatoplasties (cleft palate surgery) were performed in that period of time reflecting an overall rate of 7.7 palatoplasties per 10,000 deliveries. The authors noted that this rate is decreasing, and could be a result of a lower number of women smoking during pregnancy (among other factors) (ElHawary et al. 2021).

Risk Factors

Although the specific causes of most birth defects are not known, certain risk factors have been identified that might increase the chances of having a baby with a birth defect:

  • Smoking, drinking alcohol, or taking certain “street” drugs during pregnancy

  • Pre-existing medical conditions that are uncontrolled (high blood pressure, diabetes, hyperthyroidism)

  • Taking certain medications (women should tell their HCP all recent and current medications they are/were taking)

  • Family history of birth defects

  • The woman is older than 34

  • Body Mass Index (BMI) over 30

The presence of one or more risk factors does not mean that event will always happen.

Having one or more of these risks does not mean a woman will have a baby with a birth defect; just that an increased risk may exist, and women without risk factors may also still have a child with a birth defect.


Initial screening for birth defects was developed with the routine use of ultrasound. However, although ultrasound detection has made a significant impact on the treatment and preparation for having a baby with a birth defect, not all birth defects can be identified, or are identified correctly, with ultrasound.

One of the most common uses of ultrasound during pregnancy is to monitor the growth and development of the fetus.  It is safe, convenient, painless, and yields immediate and extensive results.

Pregnant women are advised to have their first-trimester nuchal translucency scan between 11 and 14 of weeks pregnancy, and an anatomy/anomaly scan between 18 and 20 weeks.

However, the biggest risk of ultrasound assessment is potential misdiagnosis.  Due to this, if an ultrasound exam yields “markers” or possible findings during any scan, follow up is recommended through various additional screening and diagnostic tests.

The risk of some conditions can also be determined before pregnancy through some of these same screening tests.


Not all birth defects can be prevented, but women can take steps to increase the chance of having a healthy baby:

Pregnant women should:


View the following video from the U.S. Centers for Disease Control and Prevention regarding the prevention of birth defects and what researchers have learned in the past few decades:


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