There are those who pretend that we do not need ultrasound anymore to detect fetal anomalies, “Just use maternal blood and with various forms of genetic testing and you will be able to detect the majority of fetal anomalies.”

Well, let me rebuke this insinuation.

Genetics

There is no doubt that prenatal genetic testing has come a long way from using only maternal age to assume a risk of Down syndrome (for instance 1 in 1250 at age 25 and 1 in 385 at age 35). Maternal serum screening came next. At first, levels of alpha-feto-protein (AFP) were found to be lower in mothers carrying fetuses affected with Down syndrome.

Then, other markers, such as human chorionic gonadotropin (hCG), unconjugated estriol, and dimeric inhibin A, were determined to display characteristic patterns in pregnancies with Down syndrome, with the introduction of the double, triple, and quadruple screening in the second trimester. This moved to the first trimester, with incorporation of fetal nuchal translucency (NT), pregnancy-associated plasma protein A (PAPP-A), and the beta subunit of human chorionic gonadotropin (β-hCG). A high detection rate of 85–90% was attained for Down syndrome and 90–95% for trisomy 18, with a 5% false-positive.

A combination of both the first and second trimester was introduced, to further improve the detection rate and, at the same time, decrease the false-positive rate.  In some of these tests only serum fetal-placental protein markers were considered (integrated) and in others ultrasound findings (NT) and various serum markers were combined (integrated, sequential, and contingent).   

It is widely accepted that testing of the type used nowadays originated from a Lancet paper in 1997 by Lo and colleagues, describing circulating cell-free fetal DNA (ccffDNA) in the plasma of pregnant women. It took almost 15 years for the technology to become clinically available¹. At first, it was used to determine the risk of trisomies and sex chromosome anomalies. Originally designed as noninvasive prenatal diagnosis (NIPD) or noninvasive prenatal testing (NIPT), the general opinion is that these are still screening (and not diagnostic) tests, hence the designation noninvasive prenatal screening (NIPS). I prefer noninvasive DNA screening (NIDS) because, after all, ultrasound is NIPT!

Nowadays, NIDS can be used to identify Rhesus group and some single-gene fetal conditions, autosomal dominant, recessive or sex-linked (eg, cystic fibrosis, achondroplasia, thanatophoric dysplasia, sickle cell disorder, congenital adrenal hyperplasia, spinal muscular atrophy, and hemophilia). Most conditions require using a maternal blood sample only but many require a paternal blood sample. Normal karyotype doesn’t mean everything is fine, hence chromosomal microarray, introduced in the prenatal diagnosis clinical setting in 2005. Looking for submicroscopic aberrations <5Mb can provide additional diagnostics in about 10% of fetuses with multiple anomalies¹. The latest reiteration of the technology is genome-wide monogenic NIDS².

Screening beyond the common trisomies is currently not recommended by the American College of Obstetricians and Gynecologists³. So where does ultrasound stand?

Ultrasound is alive and doing fine, thank you

In the general population, chromosomal abnormalities are less frequent than structural abnormalities. A large number of fetal structural abnormalities, especially many lethal ones, can be diagnosed in the first trimester of pregnancy, therefore, ultrasound remains an essential part of the story. Ultrasound diagnosis of fetal anomalies has now moved from the mid-second trimester (18–22 weeks) to the late first–early second-trimester (approximately 11–14 weeks). It should be noted that a repeat scan at the “classical” time (18–22 weeks) is still recommended by most.

Two major reasons for the early scan: it’s a perfect time to perform a nuchal translucency (NT) measurement and, at that stage, most structural anomalies that are already present are detectable. A few examples of what is observable include all 4 limbs and all digits, cranial anatomy, estimation of the cardiac axis, and omphalocele (which is associated with Beckwith-Wiedemann and CHARGE syndromes, limb-body stalk anomaly, and Pentalogy of Cantrell, to name a few). Amputations or other unusual cleft due to amniotic band syndrome are visible and cardiac position and orientation can also be determined. In incidences of heart defects, dextrocardia is associated with 90% and situs inversus with levocardia with over 95%.

Most of the above anomalies will be associated with an increased NT, as will pulmonary, gastrointestinal and genitourinary conditions, diaphragmatic hernia, skeletal dysplasia, fetal anemia, and abnormal lymphatic drainage⁴. A third of congenital abnormalities occurring in fetuses with increased NT may remain undetected in the first trimester of pregnancy, unless cfDNA is used in combination with fetal sonographic NT assessment. When karyotype is normal, 10% of fetuses with an increased NT (>95^th percentile) have structural abnormalities⁵.

In one study⁵, 65% of structural abnormalities would have potentially been missed in the first trimester if cfDNA had been used as a first-trimester screening test without an early ultrasound scan. Furthermore, if cfDNA only was used, besides structural defects, one third of other anomalies would have been missed: sex chromosome abnormalities, triploidy, single gene disorders, and submicroscopic aberrations <5Mb. In addition to NT measurements and detection of structural anomalies, several other sonographic markers have been described: nasal bone, ductus venosus Doppler anomalies and tricuspid regurgitation, helping to determine a high-risk group for whom genetic screening will have a high yield.

When these or/and other ultrasound-diagnosed fetal anomalies are present, whole-exome-sequencing can add relevant information in cases when an etiology could not be elucidated by fetal karyotype testing or chromosomal microarray⁶.

In a very recent article, Bedei et al. propose several conclusions, one of them being: “NIPT should always be combined with a skilled ultrasound examination.”⁷

My thoughts, exactly⁸.

I purposely do not wish to initiate a discussion on the ethical, moral, philosophical, religious, or emotional values or demerits of prenatal diagnosis. While some will say that all this is a veiled “search and destroy” exercise, others will explain that knowledge is power. Power to choose but also power to be ready when the baby is born or power to correct certain anomalies in the womb or intervene immediately at birth. Both sides of this argument may be defensible, but that is for another blog.

References

1. Talkowski ME, Rehm HL. Introduction of genomics into prenatal diagnostics. Lancet 2019 Feb 23; 393(10173):719–721.

2. Rabinowitz T, Shomron N. Genome-wide noninvasive prenatal diagnosis of monogenic disorders: Current and future trends. Comput Struct Biotechnol J 2020; 18:2463–2470.

3. American College of Obstetricians and Gynecologists screening for fetal chromosomal abnormalities: ACOG practice bulletin summary, number 226. Obstet Gynecol 2020; 136:859–867.

4.  Baer RJ, Norton ME, Shaw GM, et al. Risk of selected structural abnormalities in infants after increased nuchal translucency measurement. Am J Obstet Gynecol 2014; 211:675.e1–19.

5. Bardi F, Bosschieter P, Verheij J, et al. Is there still a role for nuchal translucency measurement in the changing paradigm of first trimester screening? Prenat Diagn 2020; 40:197–205.

6. Petrovski S, Aggarwal V, Giordano JL, et al. Whole-exome sequencing in the evaluation of fetal structural anomalies: a prospective cohort study. Lancet 2019;393(10173):758-767

7. Bedei I, Wolter A, Weber A, Signore F, Axt-Fliedner R. Chances and challenges of new genetic screening technologies (NIPT) in prenatal medicine from a clinical perspective: A narrative review. Genes (Basel) 2021; 12:501. 8. Rauch KM, Hicks MA, Adekola H, Abramowicz JS. Aneuploidy screening: the changing role of ultrasound. In: Abramowicz JS (ed). Ultrasound in the First Trimester, a Comprehensive Guide. Switzerland: Springer International Publishing AG; 2016:131–152.

Jacques S. Abramowicz, MD, FACOG, FAIUM, is a professor of OB-GYN and Director of Ultrasound Quality Assurance in the Department of Obstetrics and Gynecology at the University of Chicago.

More from Jacques Abramowicz, MD:
COVID-19: How to Prepare Yourself and Your Ultrasound Equipment During the Pandemic, an on-demand webinar from the AIUM (a collaborative activity with Samsung).

Interested in learning more about obstetric ultrasound? Check out the following posts from the Scan:

• Saving Lives With Ultrasound: How to Improve Placenta Accreta Spectrum Antenatal Detection and Management
• Pre-eclampsia, Growth Restriction, and a Placenta Bank
• Determining Umbilical Cord Blood Flow
• Dating Pearls