Birth Defects and 2nd 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

Please see the section: Birth Defects and 1st Trimester Screening under the 1st trimester heading for more details of this.

Second Trimester Screening

Second trimester screening involves drawing blood work between weeks 15 and 20 of pregnancy. Different testing can be done, but in our office, we use what is called a “quad screen.” This means that we check the blood for four different hormones. Levels of AFP, estriol, hCG, and inhibin A (all hormones produced during the pregnancy) are drawn. When these levels are combined with other information about the mother, a level of risk can be assessed to the pregnancy of the fetus having one of the following abnormalities:

1. Down syndrome (detects Down syndrome in 80% of cases)

2. Trisomy 18

3. Neural tube defects (detected in 80% of cases)

As noted above, this is only a screening test. If the results of this are abnormal, further testing and counseling may be recommended by your physician.


We offer an ultrasound around 18 weeks of pregnancy to screen for certain anatomic defects. At this ultrasound, we look more closely at the baby’s spine, abdominal wall, heart, and kidneys. We can also typically determine the baby’s sex at this visit.

Screening can be done in the first and second trimesters. Our office only currently provides second trimester screening tests. Women with an increased risk of having a baby with a birth defect include women who have a family or personal history of birth defects, women who have had a child with a birth defect or genetic problem, and women who will be over the age of 35 at delivery. If you fall into one of these categories or if you are interested in earlier screening, please call or discuss with your provider at your next appointment so that we can refer you to a specialist.

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.