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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 (a chromosome defect that causes 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.

You can view some videos here as a summary:

You can also view some videos here from Northern California Kaiser Permanente. (note that at the end of the video there is a section that is only for Northern California Kaiser Permanente)

Choices Booklet:

 

Screening Tests

Screening tests estimate the chance, or possibility, that a baby has certain birth defects. For example, a screening test may show that there’s a 1 in 200 chance that a baby has Down syndrome; or a 3 in 100 chance of having spina bifida. The two types of prenatal screening tests are the California Prenatal Screening Test and the Non-Invasive Prenatal Testing (NIPT).

California Prenatal Screening Test

All pregnant women in California are offered testing for certain birth defects through the California Prenatal Screening Program.

Three parts:

  • 1st trimester blood test
    • Blood drawn at 10 weeks 0 days to 13 weeks 6 days
  • Nuchal Translucency (NT) ultrasound of the fetus
    • Performed at 11 weeks 2 days to 14 weeks 2 days
  • 2nd trimester blood test
    • Blood drawn at 15 weeks 0 days to 20 weeks 0 days

Detects:

  • About 80-90% of Down syndrome
  • About 67-81% of trisomy 18
  • About 80% of spina bifida
  • About 85% of abdominal wall defects
  • About 60% of Smith-Lemli-Opitz syndrome (a rare inherited form of intellectual disability and birth defects)

Non-Invasive Prenatal Testing (NIPT)

NIPT is offered to women who have a greater chance of having a baby with a chromosome abnormality. It can be done between 10 and 24 weeks of pregnancy.

Usually offered if you:

  • Will be 35 years old or older at delivery
  • Have had some types of positive California Prenatal Screening test results
  • Have had some types of ultrasound abnormalities
  • Have had some types of chromosome problems in a prior pregnancy or in one of the parents

Detects:

  • About 99% of Down syndrome
  • About 97% of trisomy 18
  • About 87% of trisomy 13
  • Sex chromosome conditions (e.g., Turner Syndrome, Klinefelter syndrome)

If you have NIPT, you will be offered a blood test in the 2nd trimester (quad marker screening) to determine your baby’s chance of having spina bifida and abdominal wall defects. The blood test is performed between 15 and 20 weeks of pregnancy.

 

Diagnostic tests

Diagnostic tests confirm or rule out a specific birth defect. They are generally offered if you:

  • Will be 35 years old or older at delivery
  • Have had some types of positive California Prenatal Screening test results
  • Have had some types of ultrasound abnormalities
  • Have had some types of chromosome problems in a prior pregnancy or in one of the parents

Chorionic Villus Sampling (CVS)

  • Test removes small sample of tissue (“chorionic villi”) from the growing placenta
  • Performed at 10-13 weeks
  • Finds more than 99% of chromosome abnormalities, including Down syndrome
  • Does not check for neural tube or abdominal wall defects, so a second trimester blood test is recommended at 15-20 weeks

In rare cases when a tissue sample cannot be taken or results are not clear, your physician will recommend amniocentesis later in your pregnancy.

Complications

Minor complications after CVS such as cramping, spotting, and infection can occur.

More serious complications include miscarriage and bleeding. The chance of having a serious complication ranges between 1 in every 300 procedures and 1 in every 500 procedures. The overall miscarriage rate after CVS is slightly higher than the rate after amniocentesis.

Amniocentesis

  • Test uses an ultrasound-guided needle to remove a very small amount of fluid from the amniotic sac around your baby
  • Fetus generally replaces fluid in less than 24 hours
  • Not painful for most women
  • Performed at 15-20 weeks after 1st day of your last period
  • Finds more than 99% of chromosome abnormalities, including Down syndrome
  • Finds most neural tube and abdominal wall defects

Complications

Minor complications after amniocentesis such as cramping, spotting, slight leakage of amniotic fluid and infection can occur.

More serious complications include miscarriage, bleeding, infection, leakage of amniotic fluid or premature birth. The chance of having a serious complication ranges between 1 in every 300 procedures and 1 in every 500 procedures.