Pregnancy | Prenatal Diagnosis

Screening can provide important information about potential risks for pregnancy. Screening tests, such as a simple blood test, can identify women at increased risk of having a baby with a birth defect. These tests have no risks of miscarriage, but can't determine with certainty whether a fetus is affected.

Diagnostic tests, on the other hand, are extremely accurate at identifying certain abnormalities in the fetus, but carry a small risk -- generally less than 1 percent risk -- of miscarriage. Diagnostic tests include amniocentesis, which involves removing amniotic fluid, and chorionic villus sampling (CVS), which involves removing some tissue from the placenta.

The following are frequently asked questions about screening and diagnostic tests for genetic disorders, as well as questions regarding choroid plexus cysts that are sometimes detected by ultrasound scans.

Screening and Diagnostic Tests

Testing for Cystic Fibrosis

Choroid Plexus Cysts

Screening and Diagnostic Tests

What types of tests are available?

There are several options for both screening and diagnostic testing of the fetus. Screening tests have been designed to identify women who are at increased risk of having a baby with a birth defect. These tests have no risks of miscarriage, but cannot determine with certainty whether a fetus is affected. Diagnostic tests are extremely accurate at identifying certain abnormalities in the fetus, but carry a small (generally <1%) risk of miscarriage. At UCSF, we offer options for both screening and diagnostic testing.

What if a screening test is positive?

If you have positive results on a screening test, you can discuss what this means with a genetic counselor. Options for further diagnostic testing will be explained. The decision as to whether to have invasive genetic testing is up to you.

Who should consider diagnostic testing?

There are certain guidelines regarding who might benefit from genetic counseling and prenatal testing. These include:

What if a diagnostic test is positive?

If the diagnostic test finds a genetic abnormality, the significance of such results should be discussed with experts familiar with the condition, including a medical geneticist and a genetic counselor, as well as your own doctor. Medical geneticists and genetic counselors are available at the UCSF Prenatal Diagnosis Center. Referrals and support information are available for all decisions.

Will health insurance cover testing?

Most insurance plans cover prenatal testing especially for women over 35 years of age. You may need a referral from your primary care physician or health plan for your health insurance to cover the services. Our office staff is available to assist you with insurance questions.

Carrier testing, which is performed by a blood test, can detect gene mutations. There are over 800 genetic changes or mutations known to cause cystic fibrosis (CF) alone. A negative carrier test does not eliminate the chance you may carry a defective gene, but can lower that chance. Some people have rare mutations that cannot be detected by the routine testing available.

What is carrier testing ?

What does it mean if I am a carrier?

If you are a carrier, there is no impact on your health. However, there is a chance that you could have a child with cystic fibrosis. Your partner should undergo screening, if this has not already occurred. If only one of you is a carrier, the chance that you will have a child with CF is very low, although not zero. If you are both carriers, there is a 25 percent chance in each pregnancy to have a child who inherits two copies of the CF gene and has CF.

If both partners are carriers, it is possible to test the fetus if you are already pregnant. This can be done through chorionic villus sampling (CVS) at 10 to 12 weeks of pregnancy, or by amniocentesis at 15 to 20 weeks of pregnancy. If you are not pregnant, there are other options for preventing having a child with CF. You can meet with a geneticist and genetic counselor to discuss these issues.

If no one in my family has cystic fibrosis, is my risk to be a carrier very low?

Not necessarily. If no one in your family has cystic fibrosis, your chance of being a carrier depends on your ethnic ancestry:

If there is a family history of CF, your risk is higher, regardless of your ancestry.

If my test result is normal, could I still be a carrier?

Yes. If you have the test and have no CF mutation identified, your chance of being a CF carrier is much reduced. However, because some people have rare CF mutations that cannot be detected by routinely available testing, there is still a small chance that someone with a negative test could be a carrier and have a child with CF.

Will insurance pay for CF screening?

Each insurance company is different. It is important to check with your insurance company prior to testing, to see if your medical plan will cover the cost. Some insurance companies prefer that you use a particular laboratory, although not all laboratories screen for the same number of mutations. The more mutations tested, the higher the chance of identifying a carrier. Carrier screening costs about $150 to $200.

Choroid Plexus Cysts

The presence of isolated choroid plexus cysts (CPCs) on a second trimester ultrasound is a common cause of anxiety, although it is almost always an innocent finding. Here are the answers to some commonly asked questions about CPCs.

What are choroid plexus cysts?

The choroid plexus is the part of the brain that makes cerebrospinal fluid, the fluid that normally bathes and protects the brain and spinal column. In about 1 percent to 2 percent of normal babies -- 1 out of 50 to 100 -- a tiny bubble of fluid is pinched off as the choroid plexus forms. This appears as a cyst inside the choroid plexus at the time of ultrasound. A choroid plexus cyst can be likened to a blister and is not considered a brain abnormality.

What is going to happen to the cyst? Will it enlarge?

In the vast majority of cases, the cyst resolves or disappears and has no consequences.

What is the concern?

As mentioned, choroid plexus cysts are present in 1 percent to 2 percent of normal fetuses. However, in a very small percentage of fetuses with choroid plexus cysts, there is an associated chromosome disorder called trisomy 18. Fetuses with trisomy 18 have an extra copy of chromosome 18. Frequently, fetuses with trisomy 18 are stillborn. Survivors beyond infancy are rare. They have severe mental retardation and a variety of other problems including abnormalities of almost any organ system such as the heart, brain and kidneys. Fetuses with trisomy 18 have choroid plexus cysts about a third of the time. Therefore, when we see choroid plexus cysts, we are concerned that the fetus may have trisomy 18.

Trisomy 18 is rare. It is present in less than 1in 3,000 newborns. Choroid plexus cysts are relatively common in normal fetuses. Most fetuses with a choroid plexus cyst are normal. Furthermore, many of the abnormalities associated with trisomy 18 can be detected by a careful ultrasound. In fact, fetuses with trisomy 18 almost always demonstrate abnormalities on ultrasound in addition to choroid plexus cysts, although some of these abnormalities can be quite subtle. If no additional abnormalities are detected by a thorough "level II" ultrasound, the likelihood the fetus has trisomy 18 is very low.

What are the odds that it is a sign of the chromosomal disorder trisomy 18?

The precise rate of risk is difficult to estimate and is somewhat controversial, but most doctors believe it is well under 1 percent. In other words, a fetus with choroid plexus cysts but an otherwise normal ultrasound has a better than 99 percent chance of not having trisomy 18. A normal alpha fetoprotein (AFP) test further reduces the likelihood of trisomy 18.

Does the size of the cyst matter?

Ordinarily, the size does not matter, although multiple, large cysts are slightly more worrisome.

What is the next step after a cyst is discovered?

We can perform an amniocentesis, and count the number of chromosomes in the fetus to be certain there is not an extra copy of chromosome 18. At the same time, we can rule out other chromosomal abnormalities, such as Down syndrome, although we have no reason to suspect any of these in your case. However there is a small risk of amniocentesis causing a miscarriage.

How high is the miscarriage risk with amniocentesis?

The rate of risk for miscarriage with amniocentesis is difficult to determine. Many doctors cite 0.5 percent or 1 in 200.

Can I just wait to see if the cyst resolves?

That won't help your decision, because cysts almost always resolve in both normal fetuses, as well at those with trisomy 18. The natural course for a CPC is to resolve or disappear. However, even when the cyst resolves, it does not reduce the chance that the fetus has trisomy 18.

What should I do?

If you do nothing further, you should be confident that your baby most likely has normal chromosomes. If you have not yet had an alpha fetoprotein (AFP) test, this can be done to further evaluate the chance of trisomy 18. A level II ultrasound, or a targeted scan, should be done, if it has not been done already to look for other signs of trisomy 18. If you need to be certain, we can do the amniocentesis, with the knowledge that it is very unlikely to result in a complication. Your decision should depend on what you would do with the information from an amniocentesis, how you would feel about a miscarriage, how you would feel about an affected child and the anxiety you might feel until the child is born. If you want to proceed with the amniocentesis, you can be reassured that the complications from the procedure are quite rare.