Prenatal Genetics

Reduction of Birth Defects and Genetic Disease Risks

Before conceiving a pregnancy there are many things a couple can do to reduce the chance of birth defects and genetic disease.

  • Take Prenatal Vitamins with folic acid before getting pregnant. Additional information on preventing birth defects.
  • Talk to a doctor about screening to determine if you are a carrier of a genetic disease. Early identification of genetic diseases can affect the treatment.
  • Learn more about your family medical history on both the maternal and paternal sides of your family and your partner’s family. Write this information down and bring it to your doctor.
  • If you have concerns that a medication, drug, or other exposure could affect your future children, contact the Texas Teratogen Information Service Genetic counselors are available to provide information.

Prenatal genetic testing is one way a woman can improve the chance to have a healthy baby. About 95% of women who have prenatal genetic testing receive good news from the testing laboratory. Some of the tests that can look at the genetic health of the baby include chorionic villus sampling (CVS), amniocentesis, ultrasound, and maternal blood testing. It is important to learn all you can about the specific procedure or test itself. It is also a good idea to know its purpose, risks, benefits, and limitations. Below is a description of these procedures.

Some prenatal genetic testing like maternal blood testing is only a screening test. A screening test will tell you that there is an increased risk for a genetic disorder or birth defect. You may need further diagnostic testing to determine if there is a problem. Other genetic tests like amniocentesis will tell you if there is a specific genetic condition. These tests are all done at different weeks of the pregnancy, so you need to talk to your doctor about prenatal genetic testing before or early in the pregnancy.

Common Reasons for a Preconception / Prenatal Genetic Referral

  • Mother will be 35 years or older at delivery
  • Abnormal results from a multiple marker screen or fetal ultrasound
  • Personal or family history of a known or suspected genetic disorder, birth defect, or chromosome abnormality
  • Exposure to a chemical/drug/environmental agent that is known or suspected to cause a birth defect
  • A mother with a medical condition known or suspected to affect the development of the baby. (Ex. Seizures and diabetes)
  • Two or more pregnancy losses
  • Close biological relationship of the parents (first cousins or closer)
  • Ethnic predisposition to certain genetic conditions


Amniocentesis is a test that involves taking a small amount of amniotic fluid from the uterus or womb. This fluid contains some of the baby’s skin cells that have washed off and are floating in the fluid. The laboratory can look at these skin cells under a microscope and learn more about the health of the baby. These cells can be examined to detect chromosomal abnormalities or can be tested for certain genetic conditions through DNA testing. The risk for a baby to have a chromosome disorder increases with the woman’s age.

Mother's Age at Delivery Risk for Down Syndrome
20 1/1923
30 1/885
31 1/826
32 1/725
33 1/592
34 1/465
35 1/365
36 1/287
37 1/225
38 1/177
39 1/139
40 1/109
41 >1/85
42 1/67
43 1/53
44 1/41
45 1/32

Chorionic Villus Sampling or CVS (placental biopsy)

CVS is a test done by putting a small tube or catheter through the vagina and into the uterus to remove some cells from the placenta. It can also be done by putting a needle through the mother’s abdomen into her uterus and then into the placenta. The doctor decides the method and depends on factors such as the location of the placenta. The placenta has the same genetic makeup as the fetus, so testing these cells is a way to indirectly look at the baby’s cells. This test is done between 9 and 12 weeks of pregnancy.

Maternal Serum Screening (triple or quad screen, AFP plus, multiple marker screen)

The maternal serum screen is a prenatal screening test using the blood from the mother to identify women that may be at an increased risk of carrying a baby with specific birth defects, such as spina bifida, anencephaly, Down syndrome, and Trisomy 18. This test is usually done on the mother’s blood between the 15 to 20 weeks of the pregnancy and is recommended for all pregnant women.

Specific proteins or hormones (Alpha-fetoprotein, human chorionic gonadotropin (hCG), unconjugated estriol, and inhibin) are made by the baby or the placenta and are found in the mother’s blood. The levels of these proteins are linked with certain birth defects, such as spina bifida and Down syndrome. The blood test is a screening test, meaning it cannot identify a birth defect in the baby, it only indicates that the risk is higher and further testing is needed. A range of factors can influence the result of the blood test, such as the gestational age of the pregnancy or the presence of twins. It is recommended that women have an ultrasound to confirm the pregnancy dating. If the blood test is abnormal, further testing may be offered, such as amniocentesis or another sonogram.

Some facilities are performing maternal serum screening tests in the first trimester. This screening test looks at different hormones/proteins and is usually combined with ultrasound findings. Still, others use a combination of first and second-trimester screening. This combined testing is often called integrated screening.

Ultrasound or Sonogram

Ultrasound (also called a sonogram) is an imaging test used to look at the fetus. It can detect some major physical birth defects during the pregnancy, depending on the gestational age. The test uses sound waves and poses no risk to the baby. Ultrasound can verify how far along a woman is in her pregnancy. It also looks at fetal growth and can detect certain problems in the baby or pregnancy. It can be done at any time in the pregnancy, but it is usually done around 18 to 20 weeks to look for major physical birth defects.

Nuchal Translucency Screening

A nuchal translucency measurement is a procedure performed in the first trimester with ultrasound. The nuchal translucency is a fluid-filled space at the back of the baby’s neck. A large increase in nuchal translucency can be linked with an increased risk of a baby having certain problems like Down syndrome.

Percutaneous Umbilical Blood Sampling (PUBS, Cordocentesis, Fetal Blood Sampling)

PUBS is a procedure where a needle is inserted through the mother’s abdomen into the uterus and amniotic sac and then into the baby’s umbilical cord. A sample of the baby’s blood is removed from the umbilical cord and sent to a specialized laboratory for analysis. The procedure is usually done when a woman is at least 20 weeks pregnant and is usually reserved for special circumstances.

Carrier Testing for Common Conditions

Carrier testing is used to identify individuals who carry one copy of a non-working gene (mutation) for a certain genetic condition. Carriers are healthy and do not have the disease. However, if both members of a couple carry a copy of the same non-working gene, there is a 25% chance to have a child affected with that genetic condition.

Genetic carrier testing is commonly available. Examples of available genetic carrier testing include cystic fibrosis, sickle cell disease, thalassemia, Tay Sachs disease, Canavan disease, and familial dysautonomia. Some conditions are more common in specific ethnic populations (see chart below). Please consult with your doctor about genetic carrier testing since the number of available carrier tests continues to grow.

Population Condition Carrier Frequency
African American Sickle Cell disease 1 in 10
African American Cystic fibrosis 1 in 65
African American Beta thalassemia 1 in 75
Ashkenazi Jewish Gaucher disease 1 in 15
Ashkenazi Jewish Cystic fibrosis 1 in 29
Ashkenazi Jewish Tay Sachs disease 1 in 30
Ashkenazi Jewish Dysautonomia 1 in 32
Ashkenazi Jewish Canavan disease 1 in 40
Asian Alpha thalassemia 1 in 20
Asian Beta thalassemia 1 in 50
European American Cystic fibrosis 1 in 29
French Canadian, Cajun Tay Sachs disease 1 in 30
Hispanic Cystic fibrosis 1 in 46
Mediterranean Beta thalassemia 1 in 25

University of Texas Health Science Center in Houston
Texas Teratogen Information on Pregnancy Service (Texas TIPS)
6431 Fannin, MSB 3.147 
Houston, TX 77030

1-855-884-7248 (toll-free)

The University of Texas Health Science Center at Houston (UTHealth Houston - Texas Tips

Teratogen Information Databases: The Organization of Teratology Information Specialists has compiled fact sheets on various exposures of concern.

Texas DSHS Newborn Screening Program