Almost all moms are a little nervous when going in for a second trimester ultrasound. While we usually think of the ultrasound as a way to find out if we're having a boy or girl, the exam actually serves a much more important purpose because it helps to identify serious birth defects. I was very glad when, after weeks of anxiety, my ultrasound showed a healthy baby boy… but, unfortunately, an ultrasound isn't an absolute guarantee for a healthy baby. Here's some information you may find helpful about the use of ultrasounds to identify birth defects.
1. A routine ultrasound is highly effective in finding fatal birth defects. If your baby has a very serious abnormality that will inevitably result in an early death, such as missing or severely malformed organs, the odds of finding on a second-trimester ultrasound are extremely high, nearing 100%. One study found that 89% of babies who died of birth defects, including heart defects and spinal defects, were diagnosed with a routine prenatal ultrasound. Since the study took place in 1994, when ultrasounds were less detailed, it may be safe to assume that the chances of finding a defect are even higher now.
2. Only about half of all birth defects, including minor defects, are diagnosed before the baby is born. Overall, if your baby has a birth defect of any kind-- including very mild problems like extra fingers and clubbed feet-- your chances of identifying it on ultrasound are about 53%. One obstetric practicestates that about half of babies with Down's syndrome are identified with a routine ultrasound, as well. While a normal ultrasound is reassuring and rules out most very serious birth defects, it is by absolutely no means a guarantee.
3. A high-resolution ultrasound offers extra reassurance. If you're at a high risk for having a baby with serious abnormalities-for example, if you're over 35 or have a family history of some birth defects-- your obstetrician or midwife may refer you for an ultrasound that offers an extremely detailed image. These are highly successful in identifying problems, although specific statistics aren't yet clear, since they are a fairly new technology. If you have a high-resolution ultrasound, you can be fairly certain of your baby's health and safety if the results are normal.
4. Sometimes, an ultrasound might show a problem when there isn't one. About 5-15% of birth defects diagnosed through ultrasound are misidentified. Just as you can't interpret a "clear" ultrasound as a guarantee that your baby is healthy, you can't necessarily know that a problem on an ultrasound is definitely present and correctly diagnosed. Although you should certainly prepare for the likelihood that something is wrong, you shouldn't necessarily lose hope in the possibility that everything will be okay. Ask your obstetrician for an estimate about how accurate the ultrasound's findings are; some defects are more obvious than others.
5. Ask about other ways to screen for problems. An ultrasound isn't the only way to identify problems when you're pregnant. Around the same time as your mid-pregnancy ultrasound, you'll probably have a triple or quad screening. This is a blood test that checks levels of three to four compounds in your blood. It can find anywhere from 50-95% of birth defects, depending on the condition and its severity, and can help provide some reassurance of the findings of the ultrasound-- or it can indicate that you need some extra tests. Chorionic vilus sampling (CVS) and amniocentesis are both extremely effective in finding certain genetic problems if you're high-risk, so you may want to request one or the other of these if you're concerned.
Ultimately, it's important to hope for the best and prepare for the worst. The odds that you're pregnant with a healthy baby are high even if you are at an elevated risk, but it's important to go to your ultrasound with the understanding that it might reveal bad news. Be sure to talk to your obstetrician or midwife about any concerns you may have or any tests or exams you may have while you await your little one's arrival.