Genetic Conditions

What causes genetic defects?

When a baby is born with a birth defect, the first question usually asked by the parents is "how did this happen?" Sometimes, this question cannot be answered. 

For other birth defects, there is a known cause, which may come from either genetic and/or environmental factors. Here is some general information and terms related to the different causes of birth defects:

  • Inheritance  is used to describe a trait given to you from one of your parents, including eye color or blood type.
  • Chromosome abnormalities  are stick-like structures at the center of each cell, called the nucleus, containing your genes.
  • Single gene defects  are genes that cause disease, resulting in a birth defect.
  • Multifactorial inheritance  means many factors (multifactorial) are involved with causing a birth defect. The factors are usually both genetic and environmental.
  • Teratogens  is an agent, which can cause a birth defect. A teratogen is usually something in the environment that the mother may be exposed to during her pregnancy, including prescription medication, illegal drugs, alcohol use or a disease that the mother.

How are birth defects diagnosed?

Many birth defects can be diagnosed before birth with special tests, such as prenatal diagnosis. Chromosomal abnormalities such as Down syndrome can be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta.

Fetal ultrasound during pregnancy can also give information about the possibility of certain birth defects, but ultrasound is not 100 percent accurate because some babies with birth defects will look the same on ultrasound as those without problems.

A chromosome analysis, whether performed on a blood sample or cells from the amniotic fluid or placenta, is over 99.9 percent accurate.

Tests that help screen for birth defects, including:

  • Alpha-fetoprotein is a blood test measuring the levels of alpha-fetoprotein (AFP). This protein is released by the fetal liver and found in the mother's blood. AFP is sometimes called maternal serum AFP (MSAFP). AFP screening may be included as one part of a two, three or four-part screening, called a multiple marker screen. The other parts may include:
    • Human chorionic gonadotropin (hCG) is a hormone secreted by the early placental cells. High hCG levels may indicate a fetus with Down syndrome, which is a chromosomal abnormality leading to mental retardation and distinct physical features.
    • Estriol is a hormone produced by the placenta and by the fetal liver and adrenal glands. Low levels may indicate a fetus with Down syndrome.
    • Inhibin is a hormone produced by the placenta.
    • Chorionic villus sampling (CVS) is a prenatal test, involving samples taken from some of the placental tissue. This tissue contains the same genetic material as the fetus and can be tested for genetic problems. 

Testing is available for other genetic defects and disorders depending on the family history and availability of lab testing at the time of the procedure. In comparison to amniocentesis (another type of prenatal test), CVS does not provide information about neural tube defects, such as spina bifida.

For this reason, women who undergo CVS also need a follow-up blood test between 16 to 18 weeks of their pregnancy, to screen for neural tube defects.

  • Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid surrounding the fetus to diagnose chromosomal disorders and open neural tube defects, such as spina bifida.

Testing is available for other genetic defects and disorders, depending on the family history and availability of laboratory testing at the time of the procedure. The American College of Obstetricians and Gynecologists (ACOG) recommends amniocentesis around 15 weeks to 20 weeks of pregnancy for women who are at increased risk for chromosome abnormalities.

Women who are at risk are more than 35 years of age at delivery, or have had an abnormal maternal serum screening test, indicating an increased risk for a chromosomal abnormality or neural tube defect.

However, in some situations, amniocentesis may be performed as early as 14 weeks.

  • Ultrasound is a diagnostic technique useing high-frequency sound waves to create an image of the internal organs. Many birth defects can be detected with ultrasound.

Sometimes, birth defects are not diagnosed until physical examination of the baby is done. To confirm the physical findings, a small blood sample can be taken and the chromosomes can be analyzed. This information is important in determining the risk for that birth defect in future pregnancies.