Jeanne Faulkner 10/2007
Having children means facing an endless parade of decisions - starting at your prenatal appointments and ending sometime after college. While some are simple (red or blue shirt? pepperoni or cheese?) others - like those you'll make about testing for birth defects, carry enough weight to change lives. Pregnancy is certainly life altering but a child with birth defects affects your life in ways you can't currently imagine. During these vulnerable months, we'll help sort out the tests, choices and feelings that accompany every pregnancy.
According to the American Pregnancy Association, birth defects affect over 150,000 children every year in America. The American College of Obstetricians and Gynecologists (ACOG) defines birth defects as errors in baby's development affecting how the body looks, works, or both. Many are mild but some are severe, requiring surgery or medical treatment. Birth defects may be caused by genetic or environmental factors but over sixty percent of birth defect causes are currently not known. Sometimes birth defects are inherited through genes or chromosomes or caused by the mother being exposed to harmful agents or medications.
Traits, like hair color, height and unfortunately some health disorders are passed from parent to child through genes and chromosomes. Each cell in your body is made up of genes and chromosomes. They're like the body's manufacturing instructions or the recipe for how to make a human. Each chromosome carries many genes - half from the mother and half from the father. When a fetus is carrying chromosomes that are abnormal and will result in a birth defect, it's called a genetic disorder or chromosomal abnormality. Simply put, something went wrong with the recipe. Most occur during the first 3 months of pregnancy.
Among the many tests we do in pregnancy, there are screening and diagnostic tests that look specifically for certain birth defects and chromosomal abnormalities. Screening tests (blood tests and ultrasound) calculate the odds of having a baby with certain problems. Diagnostic tests (Amniocentesis and chorionic villus sampling,) are used when screening indicates increased odds and for women at high risk for carrying a baby with birth defects. They tell you "for sure" if your child is affected. Good news - 97% of all babies are born without birth defects.
If a screening test shows an increased risk (a positive screen) of having an affected baby, further tests may be used to diagnose the problem. Though it's certainly frightening to get a positive (abnormal) screen, it only indicates there might be a problem. Most of the time, the baby is healthy even with a positive screen. Likewise, a birth defect can occur even if the test results doesn't show a problem. Not all disorders can be found by testing. Testing isn't perfect.
What are we looking for? Neural tube (incomplete closure of the fetal spine like spina bifida or anencephaly), abdominal wall and heart defects; Down syndrome, Trisomy 18 (abnormal chromosomes causing retardation and birth defects), and cystic fibrosis (when both parents are genetic carriers).
Dr Maxine Bauer, an OB/GYN at Women's Health Today says, "Screening is all about choice and is available to all mothers. It's not mandatory. Women over age 35, those with a relative with birth defects, certain blood disorders and ethnicities are encouraged to consider testing because they are at higher risk for having a child with a chromosomal abnormality.
Dr. Bauer says, "About 30% of patients decline screening. They'll accept any issues that come along with their baby, no matter what. Some prefer not to know. 70 percent choose screening and fall into two groups: One feels even if there's something wrong with their baby, they won't do anything different. They just want to prepare and prevent the shock of finding out at delivery. The other group explores quality of life issues for themselves and their baby and considers termination. It's heart breaking every time."
Dr. Bauer clarifies, "screening is non-invasive – there's no risk to the baby. Diagnostic tests are invasive and though we're careful, there's a risk of serious side effects like miscarriage, infection and injury to mother or baby." The scariest risk, of course, is miscarriage but that only occurs with 1 out of 200 amniocenteses. That means 99.5% of the time - no miscarriage.
Diagnostic tests are primarily for when the odds of a birth defect are greater than the odds of injuring baby. For example, if screening indicates a one in 100 chance of Down Syndrome - there's a 99% chance baby is fine. If we want 100% certainty, Mom needs an amniocentesis. In this case, the risk of Down Syndrome is greater than the risks of miscarriage with amniocentesis.
First trimester screening includes a combination of blood tests and ultrasound that detect 82-87% of Down Syndrome cases. Blood tests measure two substances in Mom's blood: Pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG). The ultrasound exam, called nuchal translucency screening, measures the thickness at the back of the baby's neck. Abnormalities may indicate an increased risk of Down Syndrome or other chromosomal problems. When that happens, Dr. Bauer says, " we'll sit down together and talk about further evaluation that might include a detailed anatomical ultrasound or diagnostic testing like amniocentesis or chorionic villus sampling." It's important to realize though that even with all the high-technology available in ultrasound we can't catch all birth defects.
Chorionic Villus Sampling (CVS) is a first trimester diagnostic test (typically done between 10 and 13 weeks) that evaluates cells taken from the placenta for chromosomal abnormalities. It's 98-99% accurate but doesn't indicate how severe an abnormality might be and doesn't evaluate for neural tube defects. The advantage of CVS is that it can be done much earlier than amniocentesis giving parents an earlier opportunity to make important decisions. CVS is considered a safe procedure with very few side effects. Since it is an invasive procedure, it does carry a small risk of miscarriage. The rate of miscarriage is dependent partially on the experience of the provider.
Second trimester tests include Multiple Marker screening, MS-AFP, and the quad screen which measures several substances in Mom's blood (Alpha fetal protein, Estriol, HCG and Inhibin) that indicate Down syndrome, Trisomy 18, and neural tube defects and abdominal wall defects. If they indicate a potential problem, we'll consider diagnostic tests like an amniocentesis. With amniocentesis, a small amount of fluid and cells from the amniotic sac surrounding baby are sent to a lab for genetic testing.
Diagnostic tests carry some risks and the decision to do them is not taken lightly. It involves careful assessment and thoughtful discussion with your physician and partner. Testing tells without a doubt whether your child has specific birth defects. Depending on results, we'll either breathe a sigh of relief or shed some tears and make some tough decisions. No matter what, we're in it together and the odds are, your baby is just fine.