Our Outpatient Services

The California Prenatal Screening Program

What is the California Prenatal Screening Program?

The California Prenatal Screening Program (previously known as “the expanded AFP” program), is a sequential non-invasive serum screening program that combines two blood tests (one in the first trimester and one in the second trimester) with a first trimester ultrasound to determine whether your pregnancy is at risk for either Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), an open neural tube defect (spina bifida or anencephaly) or Smith-Lemli-Opitz syndrome (a rare inborn error of cholesterol metabolism).

Who is at risk for having a child with Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18)?

Every pregnancy is at some risk of being affected with Down syndrome (named after Dr. John Langdon  Down) or Edwards syndrome (named after John Hilton Edwards).  While most people think that only women who are 35 years of age or older are at risk for having a pregnancy affected with Down syndrome, the majority of Down syndrome  cases occur in women less than 35 years of age (because that is the age group that has the most pregnancies).  Still, per pregnancy, advancing maternal age equates to increased risk of having a pregnancy affected with a chromosome abnormality.

Logistically – how and when does this testing happen?

There are two components to The California Prenatal Screening Program that must be accomplished during specific windows of time in your pregnancy.

The first component involves a first trimester maternal blood test which measures the levels of two biochemical markers - human chorionic gonadotropin (hCG) and Pregnancy-Associated Plasma Protein A (PAPP-A) and a precise ultrasound measurement of the fetal neck called the nuchal translucency (NT).

The blood test must be performed between 10 weeks and 13 weeks 6 days of your pregnancy and the NT measurement must be performed between 11 weeks 2 days and 14 weeks 2 days of your pregnancy.   The simplest and most convenient strategy is to allow us to collect your blood at the time of your NT measurement at approximately 12 weeks.  This saves you an extra trip to the lab (travel time and waiting at the lab) and assures both the blood test and the ultrasound measurement are performed during the correct window of time in your pregnancy.  The results of the blood test and NT measurement will provide you with a preliminary risk assessment that your pregnancy is affected with Down syndrome or Edwards syndrome.  That result will be called to you approximately 7-10 days after your appointment by our genetic counsellor (that is how long it takes for the lab to analyze the blood sample).   We will also forward a copy of your results to your obstetrician for your medical record.

Alternatively, you may choose to have your blood test performed before the ultrasound exam.  If you do choose this strategy, you must have the blood obtained at least 7 days prior to the ultrasound exam and bring the test requisition form number (TRF#) – found in the upper left hand corner of the lab slip – with you to your ultrasound appointment.  This number allows us to access your blood results via the internet and provide you with your risk assessment (the so called “instant risk assessment”) at the time of your ultrasound exam.

The second component of The California Prenatal Screening Program involves a second trimester blood test which is ordered by your obstetrician between 15 and 20 weeks of your pregnancy.  The results of this blood test - which measures 4 biochemical markers - human chorionic gonadotropin (hCG), unconjugated estriol (uE3), alpha-fetoprotein (AFP) and inhibin-A are combined with the results of the first trimester blood test and nuchal translucency measurement to give a final risk assessment for your pregnancy to be affected with one of the birth defects mentioned above.  Since your obstetrician orders this test, he or she will provide you with the results of this blood test when they are available – not Diablo Valley Perinatal Associates.

How accurate are the screening tests?

The first trimester portion of the screening process will result in the detection (screen positive) of approximately 70% of cases of trisomy 21 (Down syndrome) and 59% of cases of trisomy 18 (Edwards syndrome).  By combining the second trimester blood test with the first trimester results, the detection rate increases to approximately 90% for Down syndrome , 80% for Edwards syndrome, 80% for neural tube defects, and 60% for Smith-Lemli-Opitz. 

What if I didn’t begin receiving prenatal care until after 14 weeks?

Although you will be unable to get a preliminary risk assessment by performing the first component of the screening process (the first trimester blood test and nuchal translucency), you can still have the second trimester blood test.  The detection rate of the second trimester blood test alone is lower than the complete screening process (the combination of the two blood tests and the nuchal translucency) and should not be used as a substitute screening method.

What happens if I “screen positive” for an abnormality?

You will be contacted with your results and offered an appointment with our genetic counsellor to discuss your results and options for additional screening or diagnostic testing such as chorionic villus sampling or amniocentesis.  It is important to understand that a “screen positive” result does not diagnose an abnormality.  Rather, it means you are at an increased risk for an abnormality and that you should be informed about the risks and options for additional diagnostic testing.

What happens if my test results are “screen negative”?

Screen negative results reassuring but do not guarantee that your pregnancy is not affected with an abnormality.  Regardless of whether you screen “positive” or “negative”, you will be provided with numeric risk quantifying the chances your pregnancy is affected with an abnormality.  Depending on the numeric risk, you may or may not feel comfortable with your results.  If you are not reassured by your results, you can meet with our genetic counsellor to better understand your individual results.  Additional diagnostic testing such as chorionic villus sampling and amniocentesis are reasonable options you may choose – even in the setting of a negative screening result.

What is the difference between a screening test and a diagnostic test?

A screening test will provide a result that identifies the odds a pregnancy will be affected with an abnormality, while a diagnostic test (amniocentesis or chorionic villus sampling) will provide a definitive “yes” or “no” answer.

Do screening tests identify other abnormalities?

Yes.  Although not part of the exam, numerous other abnormalities are detected at the time of the first trimester ultrasound.  This is largely due to the high resolution ultrasound equipment used to perform the first trimester ultrasound exam and the specialty trained sonographers and physicians interpreting the images.

 

 

What is the difference between a screening test and a diagnostic test such as an amniocentesis or CVS?

A screening test, such as the California Prenatal Screening Program will tell you the probability your pregnancy is affected with one of several birth defects while a diagnostic test such as an amniocentesis or chorionic villus sampling will tell you with nearly 100 percent accuracy whether your pregnancy is affected with a chromosome disorder.