Birth Defects and First Trimester Screening
While most children are born healthy, approximately 3-5% of children are born with some sort of birth defect. These defects can be very minor or severe and typically develop in the first three months of pregnancy. About 70% of the time, the cause of the defect is not known. However, known causes of birth defects can also include genetics or exposure of the mother to harmful agents or medications.
Some of the more common defects that we can screen for include:
Neural tube defect: Incomplete closure of the fetal spine that can result in spina bifida or anencephaly. The risk of this can be decreased with folic acid. This is why it is important to take folic acid prior to becoming pregnant or as soon as you find out you are pregnant.
Abdominal wall defects: This refers to an opening in the abdominal wall that allows the bowel to protrude through. Gastroschisis is a defect that describes a loss of muscle and skin on the abdominal wall. The bowel is able to protrude through this hole. Another type is when the tissue around the umbilical cord is weak and allows organs to protrude into this area (omphalocele).
Down syndrome: A syndrome that results from an extra copy of chromosome 21. Typical features of this include mental retardation, abnormal facial structure, and medical problems such as heart defects.
Trisomy 18: A syndrome that results from an extra copy of chromosome 18. This causes severe mental retardation, birth defects, and is typically not compatible with life.
A screening test is a test that assesses the risk your baby has of a certain birth defect. The advantage of this kind of test is that these tests are not invasive and do not pose a risk to you or the baby. However, because the test is used only to screen, abnormal results may require further testing.
First Trimester Screening
A first trimester screening test is a combination of two things—blood work and an ultrasound. This is performed between weeks 11 and 14 and tests only for Down syndrome and trisomy 18. This combination test detects Down syndrome in approximately 82-87% of cases. The testing consists of:
Blood work: The blood work checks for levels of two hormones, PAPP-A and hCG. The levels of these hormones are then compared to standard levels in women with similar demographic characteristics.
Ultrasound: An ultrasound is performed to measure nuchal translucency. This describes a measurement that is taken of the thickness of the tissue at the back of the baby’s neck. If this measurement is higher than expected, it may be a sign of a chromosomal (genetic) problem.
The results of the blood work are then combined with the results of the ultrasound and this calculation is combined with the mother’s age and used to assess a risk to the fetus.
What if I have an abnormal screen?
Abnormal screening tests do not always indicate an unhealthy baby. On the other hand, because it is not a diagnostic test, a birth defect can be present even if the test is normal. In most instances, there are no treatment options available, so the information obtained is strictly for your knowledge.
If your screening test is abnormal, your doctor may recommend a referral to a maternal-fetal medicine specialist for diagnostic testing. These kinds of tests are offered to women who are at risk of a genetic disorder based on history or the results of screening tests. These include amniocentesis, chorionic villus sampling, and more detailed ultrasound exams. Screening tests and diagnostic tests are available to any mother if she mother is interested, but invasive diagnostic tests are performed only at higher level centers and require referral.