Non-invasive prenatal testing (NIPS) has emerged as a high-accuracy screening test for chromosomal abnormalities in pregnancy. NIPS can be ordered as early as 10 weeks in the pregnancy and offers a higher sensitivity and a lower false positive rate than the traditional combined first trimester screening test.

INTRODUCTION

Approximately one in 150 live births are affected by a chromosomal abnormality. Although it is well understood that the risks of chromosome problems in pregnancy increase with maternal age, all pregnancies, regardless of maternal age, could be affected.¹ Hence, screening or diagnostic testing for chromosomal abnormalities should be discussed and offered to all pregnant women.

The most common chromosomal abnormality is Down syndrome, which occurs at a prevalence of approximately one in 800 live births.¹ In the last few decades, clinicians in Singapore have seen prenatal screening for Down syndrome transition from being part of the second trimester triple test to the combined first trimester screening test.

The combined first trimester screening test has been routinely offered to patients at KK Women’s and Children’s Hospital (KKH) for the past 15 years. It takes into account maternal age, maternal serum markers (free β-hCG and PAPP-A) and ultrasound markers of the fetal nuchal translucency measurement and assessment of the fetal nasal bone, to reach a 90% detection rate for Down syndrome at a false positive rate of 5%.

More recently in 2011, Down syndrome screening experienced a paradigm shift through the introduction of the analysis of cell-free DNA (cfDNA), which is derived from the placenta that circulates in the maternal blood.

This method of testing is also known as non-invasive prenatal screening (NIPS) and is often referred to by the general public by the various test names of different commercial companies (e.g., Harmony, Panorama).

Compared to the combined first trimester screening test, NIPS has a higher accuracy and is able to achieve an over 99% detection rate for Down syndrome at a 0.04% false positive rate.

WHAT IS NIPS

NIPS is an optional blood test that can be performed during pregnancy. Conditions primarily tested for through NIPS include Down syndrome, trisomy 18, trisomy 13 and chromosomes X and Y. Expanded screening for other chromosome defects may also be included.

NIPS involves the assessment of cfDNA that is derived from the placenta that circulates in the maternal blood. The cfDNA is quantified through whole genome sequencing, targeted sequencing or targeted microarray. The sample is then identified as “high risk” or “screen positive” if the proportion of DNA sequences from a particular chromosome is found to be elevated.

WHEN CAN IT BE DONE?

The amount of cfDNA in the peripheral blood of a pregnant woman is usually sufficient for analysis from as early as 10 weeks of gestation until term. This cfDNA accumulates with gestation and is cleared from the maternal circulation within hours after childbirth.

Hence, the result of NIPS for the current pregnancy would not be affected by cfDNA from any previous pregnancies.

BENEFITS OF NIPS

NIPS provides a very accurate assessment of the risks of Down syndrome, trisomy 18 and trisomy 13 in the pregnancy through the analysis of the blood of a pregnant woman, drawn via venepuncture.

Based on a recent meta-analysis of 35 relevant studies, the weighted pooled sensitivities of NIPS were 99.7% for Down syndrome, 97.9% for trisomy 18 and 99.0% for trisomy 13. The pooled false positive rate was 0.04%.²

NIPS is clearly a more accurate test with a higher sensitivity and a lower false positive rate compared to traditional methods of screening such as the combined first trimester screening test.

LIMITATIONS OF NIPS

• Despite its high accuracy for Down syndrome, trisomy 18 and trisomy 13, NIPS is still considered a screening test and false positive and negative results can still occur. There could be several reasons for false positive and false negative results including discordance between placental and fetal chromosomes, fetal mosaicism, maternal chromosomal abnormality or variation, low fetal fraction and a vanishing twin.
  
  A “low risk” or “screen negative” result through NIPS indicates a decreased risk for the condition but does not rule out the possibility of the condition in the pregnancy, while a “high risk” or “screen positive” result indicates an increased risk for the condition but is not confirmatory either. Hence, follow-up diagnostic testing through invasive testing (chorionic villus sampling or amniocentesis) may be subsequently offered for confirmatory results.

• Although NIPS tests for the common chromosomal aneuploidies, the list of chromosome problems is not exhaustive and varies between commercial companies. Furthermore, the accuracy of the conditions screened for through NIPS depends on the condition that is assessed and the platform that is used.

• Since NIPS is a blood test, it does not involve an ultrasound of the pregnancy and is thus unable to screen for structural defects or markers that may or may not be related to chromosome defects in the fetus. Hence, it is still important to offer prenatal ultrasound scans to all pregnant women to help screen for structural defects or markers, which may suggest testing beyond the few conditions that are covered through NIPS.

• Furthermore, testing of some samples through NIPS may not produce any results and be marked as “inconclusive”. This occurs in 0.03– 11% of samples² and may be due to technical or biological reasons. A subsequent redraw of the pregnant woman’s blood, in hopes of producing a result, may or may not be possible.

• Testing through NIPS has also been extended to twin pregnancies, but its performance is less extensively validated compared to that of singleton pregnancies. NIPS is also not suitable for pregnancies with a co-twin demise, a vanishing twin or an empty second sac.

Although NIPS is a highly accurate test for the common chromosomal aneuploidies, it must be offered and considered carefully in order for pregnant women to exercise their reproductive autonomy and make informed decisions about their pregnancies.

THE FUTURE OF NIPS

The feasibility of extending NIPS beyond the common aneuploidies has been demonstrated. The use of NIPS for monogenic disorders such as thalassemia is widely anticipated, especially by patients with family histories of certain monogenic disorders and who are reluctant to pursue invasive diagnostic testing. The many technical and analytical challenges associated with NIPS for monogenic disorders are currently being worked out, and hopefully with thorough validation and cautious implementation, NIPS for monogenic disorders will be an option that is clinically available to pregnant women in the future. 

REFERENCES

1. Nussbaum RL, McInnes RR, Willard HF. Principles of clinical cytogenetics and genome analysis. In: Thompson & Thompson genetics in medicine. Philadelphia (PA):Elsevier; 2016. p. 57–74.
2. Gil MM, Accurti V, Santacruz B, Plana MN, Nicolaides KH. Analysis of cell-free DNA in maternal blood in screening for aneuploidies: updated metaanalysis. Ultrasound Obstet Gynecol 2017; 50: 302–314.

Christina Choi, MS, CGC is the Senior Principal Genetic Counsellor at the Antenatal Diagnostic Centre at KK Women’s and Children’s Hospital (KKH). She joined KKH in 2012, and as a prenatal genetic counsellor with the Fetal Medicine team, uses her knowledge in medical genetics and understanding of the psychosocial impact of communication to provide genetic counselling to high-risk pregnancies. She also leads a team of nurse counsellors and oversees the execution of prenatal chromosomal/genetic testing at the Antenatal Diagnostic Centre at KKH.

Ms Choi received her B.A. in Biology and M.S. in Biotechnology from Johns Hopkins University and went on to obtain her M.S. in Genetic Counselling from Boston University School of Medicine. She continued to work in the San Francisco Bay Area as a prenatal genetic counsellor at a private obstetrics practice and was also involved in early startup days of technology company Counsyl Inc. She is board certified with the American Board of Genetic Counselling.

GPs can call the SingHealth Duke-NUS Genomic Medicine Centre for appointments at the following hotlines:
KK Women's and Children's Hospital: 6692 2984
National Cancer Centre Singapore: 6436 8288