Prenatal assessments for major chromosomal abnormalities have, like a pendulum, swung over the last 50 years between advancements in screening tests and diagnostic procedures.
In the 1960s, screening for advanced maternal age gave way to diagnostic amniocentesis. Maternal serum alpha-fetoprotein screening for neural tube defects came on the scene in the 1970s, and Down syndrome screening and chorionic villus sampling (CVS) followed in the 1980s. Better ultrasound screening markers were used in combination with biochemistry to advance first-trimester screening in the 1990s and 2000s, leading to a significant decline in diagnostic procedures. Now, free fetal DNA measurement, known as noninvasive prenatal screening, or NIPS, has entered the scene. This development, along with advances in the accuracy of diagnostic lab testing through microarray analysis, will soon lead the pendulum to swing back toward more definitive diagnostic procedures that require either CVS or amniocentesis.
Dr. Mark I. Evans
Screening tests provide us with odds adjustments, not definitive answers, and are meant for everyone. Diagnostic tests are meant to give us definitive answers, may have risks, and therefore have traditionally been done on "at-risk" patients. Fundamentally, a screening test gives us an impression, while a diagnostic test gives us harsh reality. As always, there will be trade-offs. No approach is perfect, and no one size fits everyone.
Risks beyond Down syndrome
During the prenatal period, patients will often say, "I’m concerned about having a baby with Down syndrome." What they really mean is that they’re concerned about having a baby with a serious problem, and Down syndrome is the name they know.
Serious problems – a Mendelian disorder, a multifactorial disorder, or a major chromosomal abnormality – affect 2%-3% of all births. Less serious chromosomal abnormalities affect 5%-6% of births. Although advanced maternal age is no longer the sole criterion for deciding who should be offered diagnostic testing, age still is a principal factor for risk determination.
At age 35, the chance of having a baby with Down syndrome is 1 in 380, but the chance of having any chromosomal abnormality detectable by karyotype is 1 in 190. For a 30-year-old, the chance of having a baby with any chromosomal abnormality is 1 in 380, and for a 40-year-old, the risk is 1 in 65.
With the first-trimester screening approach that combines maternal serum free beta-human chorionic gonadotropin (free beta-HCG) and pregnancy-associated plasma protein A (PAPP-A) with fetal nuchal translucency measurement, we are able to detect upward of 85% of fetuses with Down syndrome, or trisomy 21. Yet the disorder is only one of a large number of chromosomal abnormalities observed.
In a recent single-center study of more than 20,000 first-trimester screenings, 5.6% were positive for Down syndrome risk. Of those who subsequently had an amniocentesis or CVS, we found 4% had an abnormal karyotype. Interestingly, 40% of the time the abnormality was not Down syndrome, but another chromosomal abnormality. Similar analyses for trisomies 13 and 18 – the other major abnormalities targeted in first-trimester combined screening – yielded similar statistics (Prenat. Diagn. 2013;33:251-6).
All told, of the screen-positive pregnancies found to have an abnormal karyotype, at least 30% had chromosomal abnormalities outside of those for which they were screen positive. Such findings speak to the limitations of screening as opposed to diagnostic testing, and have implications for patient counseling. Patients should be counseled about the possibility of all chromosome abnormalities – not just Down syndrome.
The NIPS rollout
We have known for well over 100 years that fetal cells cross the placental barrier in small numbers, driving the development of what’s currently known as NIPS. The future of NIPS actually lies in an ever-expanding number of disorders, and will eventually end with sequencing the entire genome.
There are two main methods by which NIPS is done. The original and predominant method uses massive parallel shotgun sequencing, known as next-generation sequencing. This method involves whole-genome amplification and collects enormous amounts of information. Investigators are now attempting to direct amplification at the subchromosome level, mimicking some of what microarray analysis can do.
The second approach uses selected probes, or targeted sequencing, to focus on those sections of DNA that are of interest. Although this method may be cheaper in the short run, one drawback is that new probes will need to be created for each new disorder.
Initially, investigators attempted to isolate nucleated fetal cells from the maternal blood and use them for aneuploidy detection. However, a National Institutes of Health–funded fetal-cell isolation study that ended in 2002 reported disappointing results: Fetal-cell isolation methods had low sensitivity and other technological shortcomings. Subsequently, a number of companies attempted to replicate and improve the work, also without much success.