Introduction
Discrepant laboratory results can arise for various reasons, including laboratory errors, inherent variations in the type of sample tested, the genetic mechanism underlying the abnormality, or difference in the design of the assays used. Understanding the reason for the discrepancy is important as laboratory results often guide clinical management. This is especially critical in the prenatal setting where genetic test results significantly impact patients’ decision making.

Case Presentation
A 42-year-old G4P1021 woman presented to a genetic counseling appointment after an abnormal prenatal cell-free DNA (cfDNA) screening performed at 12 weeks gestation reported partial trisomy 18. An ultrasound performed at 12 weeks was normal and follow up testing was discussed.

 

Diagnostic Workup
The cfDNA screen performed on peripheral blood identified a 36 Mb interstitial gain involving the long arm of chromosome 18, extending from band q12.1 to q22.1. The patient presented to maternal fetal medicine at 16 weeks gestational age for a fetal ultrasound, which was normal.  Amniocentesis was performed for diagnostic testing, which included a prenatal rapid microarray. The microarray showed no evidence of a copy number gain involving chromosome 18; however, a 15 Mb terminally located region of homozygosity (ROH) on chromosome 18 involving bands q22.1 to q23 was reported. A 50-cell karyotype was performed on cultured amniocytes to look for trisomy 18 mosaicism or a marker chromosome. This analysis identified a normal karyotype in all cells.

 

Treatment and Management
Based on the diagnostic testing results, the patient was counseled on a possible mechanism of confined placental mosaicism (CPM) to explain the discrepancy between the cfDNA screen and the diagnostic microarray.  The CPM was most likely due to partial trisomy rescue of a trisomy 18 conception.  Counseling also included the possibility of autosomal recessive disease variants within the region of homozygosity, which included 68 genes. Given that intrauterine growth restriction (IUGR) is often observed in cases of CPM, the patient was scheduled for follow-up visits to monitor fetal growth via ultrasound.

 

Outcome and Follow-Up
The patient had follow-up ultrasounds at 25, 33, and 37 weeks gestation that identified progressive IUGR and short femur length in the fetus. The decision was made to deliver at 37 weeks. A female child weighing 6 lb 2 oz (15%) and length of 18 inches (21%) was delivered. The baby had no abnormalities or complications after birth.

 

Discussion
This case adds to the existing literature regarding detection of ROH on a single chromosome by diagnostic testing after abnormal cfDNA screen results.  While ROH does not result in gain or loss of genetic material, there is an increased risk for autosomal recessive disorders within the homozygous region.  Additionally, detection of ROH likely indicates a trisomy rescue event occurring during the pregnancy, which has important implications for pregnancy management.  In our case and others, the presence of CPM contributed to IUGR resulting in the decision for the patient to deliver before the fetus was full term.  Additionally, although our analysis did not identify mosaicism for a trisomy cell line in the cultured amniocytes, it is important to recognize the variability that results after trisomy rescue.  This includes low-level trisomies in other tissues not tested or the presence of small supernumerary marker chromosomes.    

 

Conclusion
This case highlights the importance of diagnostic testing in the setting of abnormal cfDNA screening, especially in the absence of ultrasound findings. In this case the diagnostic testing provided important insights into the mechanism of a likely trisomic conception and how CPM could impact the pregnancy. Diagnostic testing can identify aneuploidy, structural changes, mosaicism, and large regions of homozygosity, which can all have significant impacts on clinical decision making.