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 which can be performed during pregnancy to detect certain birth defects.


There are many different kinds of birth defects, but only a few can be diagnosed prior to the baby's birth. Chromosome abnormalities and neural tube defects are the most common birth defects that we look for with routine prenatal diagnosis procedures. Occasionally, prenatal diagnosis procedures are done to detect certain genetic diseases, such as cystic fibrosis and sickle cell disease. This specialized testing is offered when a pregnancy is known to be at-risk for a genetic disease. Sometimes, the risk is based on genetic carrier testing in the parents, other times it may be related to a family history of a genetic disease.


Prenatal diagnostic procedures include amniocentesis and chorionic villus sampling (CVS). Amniocentesis and chorionic villus sampling (CVS) can detect specific birth defects, such as chromosome abnormalities. Both procedures have a very low risk of miscarriage or loss of the baby (about 1 in 500 or less).

Deciding whether to have diagnostic testing

Some women decide to have diagnostic testing to get the most complete information about birth defects in the developing baby. Other women choose diagnostic testing only after a screening test shows a higher 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, such as whether or not the baby has a chromosome abnormality.
  • Chromosome abnormalities are not curable.
  • Prenatal diagnostic procedures have a small risk for miscarriage or loss of the pregnancy.
  • There are no prenatal diagnostic procedures that can detect all birth defects before birth.

Summary of Prenatal Diagnostic Procedures






Amniotic fluid is removed by needle

Chorionic villi are removed by catheter (transcervical) or by needle (transabdominal), depending on location of placenta



15 - 20+ weeks. Early amnio is done between 14th and 15th week.


10 - 13+ weeks.

Accuracy of Chromosomal Analysis


Greater than 99% accuracy

Greater than 99% accuracy

Neural Tube Defects (NTDs)


AFP measured in amniotic fluid, with NTD detection rate of 90 - 95%





Ultrasound done about 18-20 weeks can detect most NTDs

Does not detect NTDs.


Maternal AFP blood test offered between 15 and 20 weeks, with NTD detection rate of 80 - 85%


Ultrasound done about 18-20 weeks can detect most NTDs




About 2 weeks

About 2 weeks

Miscarriage Due To Procedure


1 in 500 or less

 1 in 500 or less

Vaginal Bleeding



Spotting may occur after transcervical CVS



Occasional, usually mild

Occasional, usually mild

Infection, Fluid Leakage

Fluid leakage may be slightly more common after early amniocentesis



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: December 2016
Reviewed by: Kaiser Permanente Genetics