Prenatal Diagnosis

Alpha-Fetoprotein (AFP) Screening

What is expanded alpha-fetoprotein (AFP) screening?

An expanded AFP screening is a simple blood test. It is recommended by the state of California for all pregnant women and can detect if they are carrying a fetus with certain genetic abnormalities such as:

• Open neural tube defects (ONTD), such as spina bifida
• Down syndrome
• Chromosomal abnormalities, such as trisomy 18
• Defects in the abdominal wall of the fetus

AFP is a substance made by the yolk sac of a fetus that enters the amniotic fluid and crosses the placenta into the mother's bloodstream. Altered AFP levels, those that are either too high or low compared to normal amounts, can indicate whether a woman is carrying a baby with a chromosome problem or certain birth defects. A pregnant woman's AFP levels decrease soon after birth.

How is the screening performed?

AFP screening is performed between 15 and 20 weeks of pregnancy. A woman will undergo a simple blood test and the blood sample will be sent off to the laboratory for analysis. In addition to checking the AFP levels, the laboratory also measures the amount of the hormones unconjugated estriol, human chorionic gonadotropin (HCG) and inhibin-A. The amount of these hormones can be altered in a woman's blood when she is carrying a baby with a chromosome problem or certain birth defects. Results are usually available within one to two weeks or less.

There is no special preparation for this test.

What does a positive screening result mean?

This is a screening test, which means that if your test result is positive, it does not necessarily mean that your baby has a birth defect, but rather that you have an increased risk of a carrying a fetus with a genetic abnormality. The purpose of this screening test is to identify those women who are at increased risk of having a baby with a birth defect as well as those who need additional testing.

If a woman does have a positive test result, her health care provider will suggest further testing to help determine whether or not there actually is a problem with the pregnancy. Women who have an abnormal expanded AFP or who are going to be 35 or older at the time of delivery have the option to undergo chorionic villus sampling (CVS) or amniocentesis. These tests can diagnose chromosomal disorders, but not all birth defects, with a high degree of certainty.

The blood test result is combined with a woman's age to estimate her own personal risk for carrying a fetus with Down syndrome, open neural tube defects, abdominal wall defects and trisomy 18. The rate of detection for women under 35 is about 85 percent for neural tube defects, 60 percent for Down syndrome and 60 percent for trisomy 18. The detection rate is higher in women over 35 years of age.

Although generally very precise, AFP screening is not 100 percent accurate. There can be false-positive results that indicate a problem when the fetus is actually healthy or false negative results that indicate a normal result when the fetus actually does have a health problem.

What does AFP screening cost?

Because AFP is recommended by the state of California for all pregnant women, most insurance companies will reimburse you for the screening test.

How do I obtain more information on AFP screening?

Ask your doctor for more information. We do not conduct screening at our offices, but provide follow-up AFP consultation and additional testing at all of our locations.

In addition to UCSF Prenatal Diagnosis Center's main San Francisco office, we have two satellite clinics in Northern California.

UCSF Prenatal Diagnosis Center
350 Parnassus Ave.
Suite 810
San Francisco, CA 94117

Monterey -- Prenatal Diagnosis Outreach Clinic
2 Upper Ragsdale Dr.
Building B, Suite 180
Monterey, CA 93940

Santa Rosa -- Prenatal Diagnosis Outreach Clinic
100 Brookwood Avenue
Santa Rosa, CA 95404

Reviewed by health care specialists at UCSF Children's Hospital.
Last updated May 8, 2007

This information is for educational purposes only and is not intended to replace the advice of your doctor or health care provider. We encourage you to discuss with your doctor any questions or concerns you may have.