Genetics Northern California

Diagnostic Procedures

Prenatal Diagnosis

Most babies are born healthy. However, every pregnant woman has a small chance of having a baby with a birth defect. Birth defects generally occur in about 3 out of every 100 births. Prenatal diagnostic procedures are medical tests that are done during pregnancy to detect certain birth defects.


Some birth defects can be diagnosed before a baby is born. Chromosome conditions and neural tube defects are the most common birth defects that can be detected by a prenatal diagnosis procedure. These procedures may also be used when genetic carrier screening or the family history shows a risk for an inherited genetic disease, such as cystic fibrosis and sickle cell disease. 


Amniocentesis and chorionic villus sampling (CVS) are diagnostic procedures done during pregnancy. Either of these procedures can diagnose specific birth defects, such as chromosome conditions. There is a very low risk of miscarriage or loss of the baby from the procedures.

Deciding whether to have diagnostic testing

Diagnostic testing gives you the most complete information about birth defects in the developing baby. Some women choose to start with this type of testing. Other women choose diagnostic testing after a screening test shows a high risk for birth defects. No matter when you are considering diagnostic testing, it is always your choice whether or not to have testing. 


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

Keep in mind:

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

Summary of Prenatal Diagnostic Procedures






Amniotic fluid is taken by a thin needle through the abdomen

Chorionic villi are taken by a thin plastic tube through the vagina (transcervical) or by a thin needle through the abdomen 



15 to 20+ 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 20 weeks detects most NTDs

Does not detect NTDs.


Blood test done between 15 and 20 weeks detects 8 out of 10 babies with NTDs 


Ultrasound done at 18 to 20 weeks detects most NTDs




About 2 weeks

About 2 weeks

Miscarriage Due To Procedure


1 in 500 or less

 1 in 500 or less

Vaginal Bleeding



Vaginal spotting sometimes happens 



Occasional, usually mild

Occasional, usually mild

Infection, Fluid Leakage



Injury To Fetus


Extremely rare

Extremely rare

Frequency of Repeat Procedure


Less than 1 in 300 (0.3%)


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: April 2020 
Reviewed by: Kimberly Barr, LCGC, Kaiser Permanente NCAL Regional Genetics