Many birth defects can be detected before birth. According to the American Academy of Family Physicians (AAFP), around 250 birth defects can be diagnosed prenatally.  Some are obvious when the infant is born or are discovered through newborn screening tests. Most birth defects are discovered in infancy, within the first year of life. Others may not be detected until childhood or even adulthood.

Prenatal screening tests are used to determine the chance that a particular fetus has a birth defect. They are safe, noninvasive and typically painless. They are not exact, but may be used to determine if prenatal diagnostic tests are necessary. Screening tests may be more accurate when performed earlier in the pregnancy. Some women may not wish to undergo prenatal screening. However, others find that these tests can give them peace of mind or time to prepare for a child with special needs.

Prenatal screening tests include:

• Maternal blood tests. Identify and measure substances in the mother’s blood that reveal information about her fetus. There are a number of these tests available, including the triple screen test and alpha fetoprotein plus, which detect different substances.
  
  
• Detailed ultrasound. Uses sound waves to create an image of the inside of a mother’s womb. This can be used to identify the physical features associated with some birth defects. Ultrasound is particularly useful in the diagnosis of structural defects affecting areas such as the heart and spinal cord.

If screening tests indicate a potential problem, a physician may recommend a diagnostic test. Prenatal diagnostic tests are around 99 percent accurate in identifying Down syndrome and other chromosomal conditions, according to the AAFP. The diagnosis of many other birth defects may not be as accurate. These tests are associated with complications, including a risk of miscarriage and preterm labor. They are typically only recommended for women with a high risk of having a child with a birth defect, including those over the age of 35 years, those with a family history of birth defects or genetic problems, and those who have had abnormal results on screening tests. Parents may use the results of diagnostic tests to prepare for the birth of a special needs child or to consider terminating the pregnancy.

Prenatal diagnostic tests include:

• Amniocentesis. With the aid of continuous ultrasound guidance, a needle is inserted through the abdomen and into the uterus. Then, a small amount of amniotic fluid (the fluid in the uterus) is removed for testing. This test is generally performed between 16 and 20 weeks into the pregnancy.
  
  
• Chorionic villus sampling (CVS). A sample of the placenta is removed for testing. It may be obtained using either a needle through the abdomen (as in amniocentesis) or a tube inserted through the vagina. This test can be performed between eight and 12 weeks of pregnancy.
  
  
• Percutaneous umbilical blood sampling (PUBS). A needle is used to remove a sample of blood from the fetus’ umbilical cord. This test carries higher risks of complications than amniocentesis or CVS and is typically used only when diagnosis must be made very quickly. It is typically performed late in pregnancy, after 20 weeks.

After an infant is born, newborn screening tests are typically performed. These tests are used to detect metabolic disorders, often called inborn errors of metabolism (IEMs), that are not otherwise apparent at birth. They typically require a small sample of blood be taken from the infant’s heel.