Birth Defects and Prenatal Testing: Choroid Plexus Cysts

Choroid plexus cysts (CPC’s) are sometimes found on ultrasound scan at around 18 to 20 weeks gestation.

Why an Ultrasound Now?

Before proceeding further it is important to understand why an ultrasound scan is offered at this time. Many people reply to this question with the answer that it is to see the baby. In addition to this, the detailed scan at this time is directed at detecting abnormalities – i.e. picking up gross defects in the baby that may or may not be compatible with life, that may require surgery once the baby is born or which might cause prolonged suffering or dependence into adult life.

Knowing about these major abnormalities gives parents the option of using this information in one of two ways. Either it allows them time to come to terms with these findings and prepare themselves for the baby’s birth, or it gives an opportunity to consider if they wish to continue with the pregnancy. Many people choose termination of pregnancy in the presence of major abnormalities.

In addition to detecting major abnormalities, ultrasound sometimes detects so-called ‘soft markers’ or findings which in themselves do the baby no harm, but may be associated with an increased risk of another abnormality. The presence of a soft marker is not diagnostic of this other abnormality, it is just a noted association in a number of cases. CPC’s are a soft marker for a gene abnormality called Edward’s syndrome.

The actual cysts themselves do not cause brain damage or other problems – ever.

What is Edward’s Syndrome?

This is a gene abnormality where the baby has extra gene material of chromosome number 18. Down’s syndrome, by comparison, is when there is extra gene material of chromosome number 21 (also called trisomy 21). Edward’s or trisomy 18 is a serious condition, often associated with other structural abnormalities, such as heart defects in around 40% of babies.

Many babies with trisomy 18 are stillborn before the end of pregnancy. Of those who are born alive, around half survive the first month of life, and 10% live to one year of age. Although serious ongoing problems are common in survivors, successful surgery to correct heart abnormalities has taken place, and survival into late teens has been reported. Links to further information on Edward’s syndrome, including a support group can be found at the bottom of the page.

This gene problem happens most often just by bad luck, though is more likely in older women, and there is a rarer familial type. A mixed variation is known as mosaic, which is when half the body’s genes are normal and half carry trisomy 18, leading to a less severe variation.

What is the Risk of Edward’s Syndrome?

If there are any other abnormalities detected on scan, then the chance of Edward’s syndrome is high (around 1 in 3) and a further test to check is advised (e.g. amniocentesis, or placental biopsy).

If there are no other abnormalities found on scan, then the risk of Edward’s syndrome is low – the individual risk is related to the woman’s age. The table at the bottom of the page lists a woman’s chance of having a baby with Edward’s by both age, and if there are CPC’s found on scan. Thus, a 31 year old woman with a baby that has CPC’s noted on scan has roughly a 1 in 264 chance that her baby has Edward’s syndrome. To put this in perspective, had this been her Down’s syndrome risk on the blood test, she would not have screened positive (but Down’s is not a universally lethal condition).

How Can I Find Out for Sure if My Baby is Affected?

This involves a test to sample the baby’s amniotic fluid (amniocentesis) or placenta (placental biopsy). Both involve a small needle being placed through the abdomen, which itself is not usually too uncomfortable. Both are carried out with ultrasound guidance.

With amniocentesis, a small amount of fluid is removed and the baby’s skin cells are cultured. With the placental biopsy, some tissue is sampled. Occasionally there is a failure of the culture to grow (about 1/200) and it will need to be repeated. Amniocentesis takes about 11-14 days for a result, but placental biopsy only 48-72 hours. Very rarely the gene make-up of the placenta is not the same as the baby and a misleading result occurs.

The major drawback of both these diagnostic procedures is the associated risk of miscarriage. For amniocentesis there is an increased pregnancy loss rate of about 0.5% and for placental biopsy 1-2%. Of course, when the risk of the condition causing concern is low, most of these miscarriages will be of normal (unaffected) pregnancies.

Only you and your partner can decide if you feel that the chance of Edward’s syndrome is high enough to accept this procedure-related miscarriage risk. Some women faced with a 1/100 risk see it as a 99% chance of being normal and count this as relatively low. Others are less satisfied even with a 1/2000 chance. Some use the miscarriage risk as a guide and say that they’ll have amniocentesis only if the risk of having an affected pregnancy is greater than the risk of miscarriage.

At the end of the day you have to live with the outcome of any decision. No one is happy when a pregnancy miscarries, particularly following amniocentesis, but some can rationalise this remote possibility, and accept that the decision they took was in good faith at that time and for the best. Some feel the risk of miscarriage of a normal pregnancy is too much to take under any circumstance and that if the pregnancy is indeed affected, then nature will take its course. All are equally valid views.

What About the Cysts Themselves?

CPC’s are very small and the risk of Edward’s syndrome seems to be no greater if they are large or small, one-sided or on both, persist later or not. Most cysts resolve by 24 to 28 weeks and a further scan can be arranged to reassure you of this. As mentioned above, the cysts do not cause brain damage or structural brain abnormalities. They are just a developmental ‘phase’ which passes. There are no implications for infant development or IQ.

More information about Edward’s syndrome can be found on the Support Organisation for Trisomy (SOFT) page at: www.trisomy.org