Almost every pregnant woman has at least one ultrasound scan during her pregnancy, and most OB/GYNs offer women a routine scan sometime in the middle of the second trimester (around 16 to 20 weeks) to check the baby's measurements and screen for signs of obvious problems. But although the ultrasound results are relatively reliable, a scan showing no problems is not a guarantee that there aren't any problems, and now and then a scan showing red flags for birth defects will turn out to be a false alarm.
Reliability for Detecting Birth Defects
Research shows that ultrasounds will catch about 74% of major birth defects, and possibly a higher number when conducted by a well-trained specialist -- and that may be higher for some types of unmistakable malformations, such as anencephaly. But other birth defects may be less obvious with the grainy images returned by a typical ultrasound and may instead become apparent at birth.
Thus, although an ultrasound showing no signs of problems is definitely a good sign and means the chances are high that everything is fine, it is not a complete guarantee that the baby will be born free of health problems.
In addition, the skill of the practitioner plays a major role in the accuracy of ultrasound. A perinatologist or a very experienced sonographer may have a better chance at catching signs of problems than a less experienced ultrasound operator.
False Positive Diagnoses of Birth Defects
Similarly, ultrasound is not always 100% accurate when it does show problems. A large study called the Eurofetus Project found that the rate of false diagnoses of birth defects was about 18%. About half of those were discovered as incorrect in subsequent ultrasounds prior to birth, with others not being discovered until after birth. Major malformations appear to be less likely to have false positive diagnoses than minor ones.
In other cases, the ultrasound may raise concern of problems but not offer enough information for a diagnosis. For example, an ultrasound may show red flags that suggest that the baby might have a condition such as Down Syndrome -- but the red flags might be false alarm. When this happens, the doctor will usually recommend an amniocentesis to analyze the baby's chromosomes before making a diagnosis. Amniocentesis has near 100% accuracy for diagnosing chromosomal problems (barring lab error, which is rare).
Gonçalves LF, Jeanty P, Piper JM. "The accuracy of prenatal ultrasonography in detecting congenital anomalies." American Journal of Obstetrics & Gynecology 1994. 171(6):1606-12.
Grandjean, Helene MD; Larroque, Daniele MD; Levi, Salvator MD. "The performance of routine ultrasonographic screening of pregnancies in the Eurofetus Study." American Journal of Obstetrics & Gynecology 1999. 181(2):446-54.
Martinez-Zamora, M. Angels, Antoni Borrell, Virginia Borobio, Anna Gonce, Marimer Perez, Francesc Botet, Alfons Nadal, Asteria Albert, Bienvenido Puerto, and Albert Fortuny. "False positives in the prenatal ultrasound screening of fetal structural anomalies." Prenatal Diagnosis Dec 2006. 27(1): 18-22.