Genetics Northern California

Diagnostic Procedures

Prenatal Diagnosis

Most babies are born healthy. However, birth defects happen in about 3 out of every 100 births. Some birth defects can be diagnosed before a baby is born. 


Prenatal diagnostic procedures are medical tests done during pregnancy to find certain birth defects. They include amniocentesis and chorionic villus sampling (CVS). There is a very low risk of miscarriage or loss of the baby from either of these procedures.

Chromosome conditions are the most common birth defects that are found by a diagnostic procedure. These procedures may also be done when genetic carrier screening or the family history shows a risk for an inherited genetic disease, such as cystic fibrosis and sickle cell disease. 

 

Deciding whether to have diagnostic testing

Diagnostic testing gives you the most complete information about birth defects in the developing baby. It is always your choice to have a diagnostic test. You can choose to start with this type of testing or ask for diagnostic testing if a blood test or ultrasound shows a high risk for birth defects. 

  

The decision to have a diagnostic test is personal. It can help to talk with your doctor, nurse practitioner, nurse midwife, or genetic counselor.

Keep in mind:


  • Diagnostic testing can give you information about the baby's health before birth.
  • Knowing about a chromosome condition (or other birth defects) during pregnancy may help you make decisions about medical care.  
  • Diagnostic procedures have a small risk of miscarriage or loss of the pregnancy.
  • Diagnostic testing does not detect all birth defects or genetic conditions. 

Summary of Prenatal Diagnostic Procedures

  

 

Amniocentesis

CVS

Procedure

Amniotic fluid is taken by a thin needle through the abdomen, without touching the baby

Chorionic villi are taken by a thin plastic tube through the vagina (transcervical)
or by a thin needle through the abdomen, without touching the baby 

 

Timing

15 to 22+ weeks. 


10 to 13+ weeks.

Accuracy of Chromosomal Analysis

 

Greater than 99% accuracy

Greater than 99% accuracy

Neural Tube Defects (NTDs)

 

Detects 9 out of 10 babies with NTDs (using AFP from the fluid)

 

  

 

Ultrasound done at 18 to 22 weeks detects most NTDs

Does not detect NTDs.

 

Blood test done between 15 and 21 weeks detects 8 out of 10 babies with NTDs 

 

Ultrasound done at 18 to 22 weeks detects most NTDs

 

Results

 

About 2 weeks

About 2 weeks

Miscarriage Due To Procedure

 

About 1 in 1000

About 1 in 1000

Vaginal Bleeding

 

Rare

Vaginal spotting sometimes happens 

Cramping

 

Occasional, usually mild

Occasional, usually mild

Infection, Fluid Leakage

Uncommon

Uncommon

Injury To Fetus

 

Extremely rare

Extremely rare

Frequency of Repeat Procedure

 

Less than 1 in 300 (0.3%)

3-5%

Restriction of Activities

Limit physical activity for 24 hours after procedure (no sexual intercourse, strenuous exercise)

Limit physical activity for 24 hours after procedure (no sexual intercourse, strenuous exercise)

Last reviewed: October 2022  
Reviewed by: Kimberly Barr, LCGC, Kaiser Permanente NCAL Regional Genetics