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Genetics

Genetics
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During Pregnancy

During your pregnancy, you can choose to have screening and/or diagnostic tests that check for birth defects, including:

  • Down syndrome (a chromosome defect that causes intellectual disability)
  • Trisomy 18 and Trisomy 13 (chromosome defects that cause intellectual disability and other severe problems)
  • Neural tube defects (an opening in the spine or skull)

Prenatal Screening tests

  • Are optional and voluntary
  • Estimate the chance of a birth defect
  • Are done on a small sample of mother’s blood
  • May be followed by diagnostic tests if needed

Diagnostic tests

  • Are optional and voluntary
  • Verify a specific birth defect
  • May be done instead of screening tests, especially if your baby has an increased chance of having certain birth defects

If screening or diagnostic genetic test results are not normal, you will be offered an appointment with a genetic counselor to discuss your options.

Pamphlet from Your Southern California Permanente Medical Group:

Screening Tests

Screening tests tell you if there is a higher chance, or possibility, that your baby has certain birth defects. They cannot tell you for sure if your baby has a problem. If your screening test shows a higher risk, you will meet with a genetic counselor who will help you decide whether you want to have a diagnostic test (CVS or amniocentesis). Genetic counselors are health care professionals specially trained in inherited conditions, birth defects, and testing for these conditions.

All pregnant patients are offered cell-free DNA testing (cfDNA, also known as Non-Invasive Prenatal Testing or NIPT) at 10 to 24 weeks gestation as part of the California Prenatal Screening Program. The cfDNA test checks the mother’s blood for extra DNA pieces from the developing baby. Extra DNA is a sign that the pregnancy has a much higher chance for certain birth defects. cfDNA finds most pregnancies with Down syndrome, Trisomy 18 and Trisomy 13.  These conditions do not usually run in families.  People of all ages, races, and incomes can have a baby with one of these problems.

The test will also screen for sex chromosome abnormalities, which are conditions that do not typically cause the same level of intellectual disability seen with Down syndrome, and may cause infertility and other clinical problems.

Between 15 and 21 weeks, you will be offered a second blood test to measure maternal serum alpha-fetoprotein (MSAFP) to test for spina bifida and abdominal wall defects.

The screening result can be positive (abnormal) even if the baby does not have Down syndrome or another birth defect.   Rarely, the screening result is negative (normal), but the baby does have Down syndrome or another birth defect.

California Prenatal Screening Program, Dept. of Public Health

All pregnant women in California are offered testing for certain birth defects through the California Prenatal Screening Program.  At your Southern California Permanente Medical Group, you do not need to sign a consent form for the screening test, but always let your doctor’s team know if you do or do not want the test. Here is some information from the California State Website:

Diagnostic tests

Diagnostic tests are usually offered:

  • When there is an abnormal blood screening test (cfDNA or MSAFP)
  • When a birth defect is suspected during an ultrasound exam
  • If either parent has already had a child with a chromosome disorder or a neural tube defect
  • When the baby is at an increased risk for a known genetic or chromosome condition
  • To any pregnant patient who desires it

Chorionic Villus Sampling  (11 to 14 weeks of pregnancy)

  • Finds more than 99% of chromosome abnormalities, including Down syndrome
  • Does not find neural tube defects, so the MSAFP blood test is recommended at 15 to 21 weeks
  • Using ultrasound as a guide, the doctor removes a small sample of tissue called “chorionic villi” from the placenta
  • Chance of any complication, including the rare chance of miscarriage, is less than 1%

Amniocentesis (16 to 20 weeks of pregnancy)

  • Finds more than 99% of chromosome abnormalities, including Down syndrome
  • Finds most neural tube and abdominal wall defects
  • Using ultrasound as a guide, the doctor inserts a thin needle to withdraw a very small amount of fluid from the uterus
  • Chance of any complications, including the rare chance of miscarriage, is less than 1%.