We are in the midst of a significant shift in our approach to prenatal screening for Down syndrome and other major chromosomal abnormalities. We now know, without doubt, that first-trimester screening that combines maternal serum free β-human chorionic gonadotropin and pregnancy-associated plasma protein-A with fetal nuchal translucency measurement is better than second-trimester screening, and we must embrace this new knowledge.
A host of studies, conducted at the front lines of clinical practice as well as at major medical centers, has provided more-than-sufficient evidence that first-trimester screening is ready for implementation in obstetric practice. Major organizations—such as the American College of Obstetricians and Gynecologists, the Society for Maternal-Fetal Medicine, and the National Institutes of Health—are supportive of the shift. For our own patients and as part of an overall public health strategy, we should be rapidly moving away from second-trimester screening and away from using advanced maternal age as the main criterion for deciding who is at risk; instead, we should move toward incorporating the tests and referrals necessary for effective first-trimester screening.
In our practice, we see hundreds of patients who, in previous pregnancies, did not learn until the 18th, 19th, or 20th weeks that they had a baby with a serious problem. They had to go through emotionally wrenching experiences regardless of what they chose to do. With their new pregnancies, they want answers, privately and urgently.
Giving our patients the opportunity to get information and make decisions privately, at a time when their pregnancies are not yet obvious, was always one of the principal driving forces behind the desire for earlier prenatal screening. Now, with first-trimester screening a reality, obstetricians need to be even better prepared to counsel patients and to accept and fully respect their various opinions and decisions.
To effectively incorporate first-trimester screening into practice, we face two other responsibilities. First, we must appreciate—and reassure patients of—the fact that, when done by experienced physicians and at the proper gestational age, chorionic villi sampling (CVS) is safe. When it is done beyond 9 weeks' gestation, the incidence of birth defects after CVS is the same as it is in patients who had no procedure. Second, we must ensure the quality of obstetric ultrasound and, specifically, the measuring of nuchal translucency.
The Road to First-Trimester Screening
We have come a long way in the past 40 years. Once, the best we could do was tell a woman that, as she got older, she had an increased risk of having a baby with a chromosomal abnormality. Then we began to understand that levels of risk were generally clumped together into 5-year cohorts, with a big jump in risk occurring between the 30− to 34-year-old cohort and the 35− to 39-year-olds. As we looked further, we saw that the slope of the curve begins to go up at about age 32 years.
Once amniocentesis was developed, it evolved from a procedure offered only to women at the very highest risk—mainly those who were older than 40 or who had a child with an abnormality—to one that was offered widely to women older than 35 years. In the 1980s, however, it took almost a month for results to come back. By that time, at 21–22 weeks, patients were visibly pregnant, and the bonding process had accelerated.
The angst faced by women at this point in their pregnancies led to the notion of trying to move prenatal diagnosis into the first trimester with CVS. By the end of the 1980s, the procedure was deemed safe and effective. We were stymied, unfortunately, by the limb reduction scare of the early 1990s—an assertion that babies born after CVS had a higher risk of certain limb defects. When this procedure is done in experienced hands and later than 9 weeks' gestation, however, the procedure carries no such risks. The quality of chromosomal study with CVS, moreover, is virtually identical to that with amniocentesis. The risk of miscarriage is also the same.
If all we did was offer CVS and amniocentesis to women aged 35 years and older, however, we would detect only about one-third of the babies born with chromosomal anomalies like Down syndrome. Significantly more pregnancies occur among younger women, and the vast majority of chromosomal abnormalities therefore occur in this “low-risk” group. For this reason—and in an effort to avoid invasive procedures when possible and when desired among older women—physicians and patients clamored for an effective screening test.
Our first obstetric prenatal screening test—the measurement of maternal serum alpha fetoprotein (AFP)—enabled us to detect about one-third of the chromosomal anomalies in women under 35.