A 37-year-old pregnant woman presents to the obstetrics and gynecology office for her initial prenatal appointment. She is currently at 9 weeks and 5 days gestation, according to her last menstrual period. Her obstetric history is significant for a normal spontaneous vaginal delivery at term 7 years prior, with no prenatal or postnatal complications.

At presentation, the patient is concerned because she is considered to be of advanced maternal age and has read that her pregnancy is considered geriatric. She is worried that she has a high-risk pregnancy and asks whether she needs genetic testing because of her age.  

Her provider explains the risks, benefits, alternatives, and limitations of the different screening options and the patient decides to undergo cell-free DNA prenatal screening at 11 weeks gestation. The patient chose cell-free DNA screening because of ease of testing and availability of results within 2 weeks. The rapid turnaround of results allows the patient and her family to have information back within the first trimester and allows them to better prepare if any abnormalities are detected. All results are normal.


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The patient has her comprehensive fetal anatomy ultrasound at 20 weeks as recommended and no structural anomalies are detected. 

Discussion

This scenario has become increasingly common in the United States as patients are having children later, often well into their 40s.1 Although the risk for fetal aneuploidy increases with maternal age, fetal chromosomal anomalies can affect patients at any age and are not related to race or ethnicity. In fact, most children with trisomy 21 (Down syndrome) are born to younger patients because of the higher pregnancy rate in younger patients.2,3 Thus, the American College of Obstetricians and Gynecologists (ACOG) and Society for Maternal-Fetal Medicine (SMFM) recommend that prenatal genetic screening be offered to all pregnant women regardless of age or risk for chromosomal abnormality.3  

Nurse practitioners and PAs should be aware of the different prenatal screening options as well as the benefits and limitations of each screening modality to ensure appropriate patient education, informed consent, and shared decision-making.

Collectively, the screening modalities in Table 1 are referred to as noninvasive prenatal screenings.3-9 Invasive diagnostic prenatal testing, including chorionic villus sampling and amniocentesis, have gestational age limitations and risk for miscarriage when performed. A review of these invasive procedures is beyond the scope of this article.

Table 1. Noninvasive Prenatal Screenings3-9

First-trimester aneuploidy screening                           
Nuchal translucency• Sonographic test that measures the fluid-filled space on the dorsal aspect of the fetal neck
• The greater the thickness, the more likely a chromosomal aberration will be found
• Normal range = 1.2 mm-1.9 mm 
• Abnormal range = ≥3 mm above the 99th percentile based upon the crown-rump length
• Used as a component of FTAS and also is independently associated with fetal aneuploidy and structural malformations (eg, cardiac anomalies, abdominal wall defects, diaphragmatic hernia)
PAPP-A• This protein is produced by the placenta and measured in maternal serum; low levels are linked to fetal aneuploidy as well as adverse pregnancy outcomes
beta-hCG• This protein is produced by the placenta and measured in maternal serum to detect fetal aneuploidy
Second-trimester quad screening
AFP• This protein is produced by the fetal liver and yolk sac
• Measurement of this protein in maternal serum is used to detect a neural tube defect (higher than normal levels) or trisomy 21 (lower than normal levels)
• This test also may be used in FTAS depending on the laboratory used3
DIA• This hormone is made by the placenta
• Elevated levels in maternal serum are associated with a higher incidence of fetal aneuploidy
uE3• Estriol is produced and released by the placenta and is measured in maternal serum as uE3
• Decreased levels are associated with a higher incidence of chromosomal abnormalities
AFP, alpha-fetoprotein; beta-hCG, beta-human chorionic gonadotropin; DIA, dimeric inhibin A; FTAS, first-trimester aneuploidy screening; quad screening, quadruple marker screening; PAPP-A; pregnancy-associated plasma protein A; uE3, unconjugated estriol

Traditional First-Trimester Aneuploidy Screening

First-trimester aneuploidy screening (FTAS) combines a maternal serum blood test (hCG and PAPP-A) with ultrasound imaging (nuchal translucency) to determine the risk for 3 of the most common fetal aneuploidies: trisomy 13, 18, and 21.2-4 An advantage of traditional FTAS is that testing can be performed early in pregnancy, between 10 to 13 weeks gestation.3

A disadvantage of FTAS is that it does not determine the risk for neural tube defects and is only used to assess the risk for fetal chromosomal abnormalities. Additional diagnostic testing is offered if a patient is determined to be at high risk based on FTAS results.2,4 Patients who consent to undergo diagnostic testing can be offered chorionic villus sampling between 10 to 13 weeks gestation or amniocentesis starting at 15 weeks gestation.7 Table 2 provides a summary of screening tests by gestational age.

Second-Trimester Quadruple Marker Screening

Quadruple marker screening (quad screen) is performed in the second trimester between 15 to 22 weeks gestation.3 This prenatal screening uses beta-human chorionic gonadotropin (beta-hCG), alpha-fetoprotein (AFP), dimeric inhibin A (DIA), and unconjugated estriol (uE3) in conjunction with a full anatomy ultrasound performed to determine maternal risk for chromosomal and other fetal abnormalities.3 Additional diagnostic testing with amniocentesis is offered if a patient is determined to be at high risk.10

Combined First Trimester and Second Trimester Screening

Integrated Screening

Integrated screening involves FTAS including NT plus quad screen.3,10 An advantage to integrated screening is that it is a more comprehensive testing and has a higher detection rate of fetal chromosomal abnormality than either the FTAS or quad screen alone and a lower rate of false positives.3 A disadvantage is that the patient does not find out the risk for chromosomal abnormalities until the second trimester as the FTAS screening results are not provided until the quad screen results are completed.3,5

Another option called serum integrated screening involves use of first-trimester serum tests, but not NT, and quad screen.3 This option has a lower detection rate than integrated screening and a similar detection rate compared with FTAS alone.3 It may be best used in locations where an NT measurement by a certified ultrasonographer is not available or when imaging is not successful because of fetal position, maternal body size, or imaging properties preclude use of NT.3

Sequential Screening

Sequential pregnancy screening also combines the FTAS and the quad screen but offers the advantage of providing FTAS results in the first trimester followed by quad results in the second trimester.3

Cell-Free DNA Screening

The newest noninvasive prenatal screening option is cell-free DNA (cfDNA). The presence of fetal cell-free DNA in maternal blood was first identified in 1997.11 Ten years later, it was discovered that cfDNA originated from placental trophoblastic cells.12 Cell-free DNA screening considers the percentage of fetal DNA circulating in maternal blood from cells undergoing programmed cell death. This measurement is called the fetal fraction. The fetal fraction varies but generally ranges from 3% to 13% of the total cfDNA in maternal blood.3,13 A minimum fetal fraction of 4% is required for accurate testing; otherwise, the reliability of the test may be significantly altered.13,14

Cell-free DNA is recommended as a first-line option in the latest ACOG/SMFM guidelines showing that it has the highest detection rate for the most common fetal aneuploidies.3 It is also the only noninvasive screening option for the identification of sex and sex chromosome fetal aneuploidies.3

In singleton pregnancies, cfDNA can identify more than 99% of trisomy 2, 99% of trisomy 13, and 98% of trisomy 18 with a false-positive rate of 0.13%.3 The negative predictive value is more than 99% in patients between age 20 and 45;3,15,16 thus, it is the most sensitive and specific screening option for trisomy. The test can be performed after 9 to 10 weeks (depending on the laboratory) until term, making it a more flexible option than other screening tests because there is no specific upper gestational age limit. Patients who screen positive should be referred for genetic counseling and offered invasive testing for diagnosis.3

While the utility of cfDNA in singleton pregnancies is well established, ACOG/SMFM recommends ultrasound imaging prior to testing for assessment of viability and determination of gestational age.3 Patients who are less than 9 weeks gestation may have a test failure. An embryonic pregnancy, multiple gestation, or observable fetal anomaly will also affect test result interpretation.3 Patients who have undergone organ transplantation should not undergo cfDNA for sex chromosome results because the donor’s DNA will be present. In addition, patients who are over 250 lb may have a test failure due to an increased amount of maternal DNA that can affect the calculations.3,16 False-positive results can occur in patients with placental mosaicism, twin pregnancy with 1 demise, maternal malignancy, and abnormal maternal karyotype.16

Table 2. Summary of Prenatal Screening Tests3

Gestational AgeLaboratory ScreeningImagingFollow-up NeededSpecial Considerations
>9-10 weeks to termCell-free DNANoneNegative: Second-trimester comprehensive fetal anatomy ultrasound
Positive: Genetic counseling, offer invasive diagnostic testinga  
• Lowest false positive rate, highest detection rate of aneuploidies
• Test failures, false positives can occur in multiple scenarios
• Can be drawn as early as 9 weeks
• No upper gestational age limits
• The only serum test approved for fetal sex identification
10 weeks-13 weeks, 6 daysFirst-trimester screening: hCG, PAPP-A  Nuchal translucency via ultrasoundNegative: Second-trimester fetal anatomy ultrasound
Positive: Second-trimester comprehensive fetal anatomy ultrasound + AFP and genetic counseling
• Requires accurate measurement of gestational age
15-22 weeksSecond-trimester quad screening: hCG, AFP, uE3, DIA  Comprehensive fetal anatomy ultrasoundNegative: Second-trimester comprehensive fetal anatomy ultrasound
Positive: Genetic counseling, Second-trimester comprehensive fetal anatomy ultrasound, offer invasive diagnostic testinga
• Lowest detection rate of all modalities when used alone
aInvasive diagnostic testing includes chorionic villus sampling and amniocentesis
AFP, alpha-fetoprotein; beta-hCG, beta-human chorionic gonadotropin; DIA, dimeric inhibin A; quad screening, quadruple marker screening; PAPP-A; pregnancy-associated plasma protein A; uE3, unconjugated estriol

Conclusion

Prenatal genetic screening should be discussed and offered to all pregnant patients regardless of age or risk for chromosomal abnormality.3 Patients who consent to prenatal screening should undergo 1 screening approach and should not have multiple screening tests performed at the same time.3 Cell-free DNA is the most sensitive and specific approach but has the potential for false-positive and false-negative results and is not equivalent to diagnostic testing.3 Patients with positive screening tests should undergo genetic counseling and comprehensive ultrasound evaluation and be offered diagnostic testing to confirm the results.3 Second-trimester ultrasound is recommended for all patients to detect fetal structural defects that may occur regardless of whether or not fetal aneuploidies are present.3

Jenna Rolfs, DMSc, MPAS, PA-C, currently serves as the program director for the University of Lynchburg PA Medicine program in Lynchburg, VA.  Dr. Rolfs has been practicing clinically as a PA since 2010 and serves as the secretary on the board of directors for the Virginia Academy of PAs. Elyse Watkins, DHSc, PA-C, DFAAPA, is an associate professor for the University of Lynchburg DMSc program in Virginia. Dr. Watkins also teaches the Women’s Health module at the University of Lynchburg PA Medicine program, and is an assistant professor at Florida State University School of Physician Assistant Practice where she teaches the women’s health module and professional development.

References

1. Lean SC, Derricott H, Jones RL, Heazell AEP. Advanced maternal age and adverse pregnancy outcomes: a systematic review and meta-analysis. PLoS One. 2017;12(10):e0186287. doi:10.1371/journal.pone.0186287

2. Shiefa S, Amargandhi M, Bhupendra J, Moulali S, Kristine T. First trimester maternal serum screening using biochemical markers PAPP-A and free β-hCG for Down syndrome, Patau syndrome and Edward syndromeIndian J Clin Biochem. 2013;28(1):3-12. doi:10.1007/s12291-012-0269-9

3. Rose NC, Kaimal AJ, Dugoff L, Norton ME; American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics; Committee on Genetics; Society for Maternal-Fetal Medicine. Screening for fetal chromosomal abnormalities: ACOG Practice Bulletin, Number 226. Obstet Gynecol. 2020;136(4):e48-e69. doi:10.1097/AOG.0000000000004084

4. Salman Guraya S. The associations of nuchal translucency and fetal abnormalities; significance and implications. J Clin Diagn Res. 2013;7(5):936-941. doi:10.7860/JCDR/2013/5888.2989

5. Lau H, Amarasekara C, Uppal T. Low PAPP-A: what are the clinical implications? Australas J Ultrasound Med. 2012;15(1):26-28. doi:10.1002/j.2205-0140.2012.tb00139.x

6. Alpha-Fetoprotein (AFP) Test. MedlinePlus. National Library of Medicine. Updated July 30, 2021. Accessed September 1, 2021. https://medlineplus.gov/lab-tests/alpha-fetoprotein-afp-test/

7. Prefumo F, Sairam S, Bhide A, Thilaganathan B. First-trimester nuchal translucency, nasal bones, and trisomy 21 in selected and unselected populations. Amer J of Obstet and Gynecol, 2006;194 (3):828-833. doi.org/10.1016/j.ajog.2005.09.008

8. Shah KH, Anjum A, Nair P, Bhat P, Bhat RG, Bhat S. Pregnancy associated plasma protein A: an indicator of adverse obstetric outcomes in a South India population. Turk J Obstet Gynecol. 2020;17(1):40-45. doi:10.4274/tjod.galenos.2020.05695

9. Practice Bulletin No. 162: Prenatal diagnostic testing for genetic disorders. Obstet Gynecol. 2016;127(5):e108-e122. doi:10.1097/AOG.0000000000001405

10. Carlson LM, Vora NL. Prenatal diagnosis: screening and diagnostic toolsObstet Gynecol Clin North Am. 2017;44(2):245-256. doi:10.1016/j.ogc.2017.02.004

11. Lo YM, Corbetta N, Chamberlain PF, et al. Presence of fetal DNA in maternal plasma and serum. Lancet. 1997;350:485-487. doi:10.1016/S0140-6736(97)02174-0

12. Alberry M, Maddocks D, Jones M, et al. Free fetal DNA in maternal plasma in anembryonic pregnancies: confirmation that the origin is the trophoblast. Prenat Diagn. 2007;27(5):415-418. doi:10.1002/pd.1700

13. Ashoor G, Syngelaki A, Poon LC, Rezende JC, Nicolaides KH. Fetal fraction in maternal plasma cell-free DNA at 11–13 weeks’ gestation: relation to maternal and fetal characteristics. Ultrasound Obstet Gynecol. 2013;41:26-32. doi:10.1002/uog.12331

14. Hu P, Liang D, Chen Y, et al. An enrichment method to increase cell-free fetal DNA fraction and significantly reduce false negatives and test failures for non-invasive prenatal screening: a feasibility study. J Transl Med. 2019;17(1):124. doi:10.1186/s12967-019-1871-x

15. Gil MM, Accurti V, Santacruz B, Plana MN, Nicolaides KH. Analysis of cell-free DNA in maternal blood in screening for aneuploidies: updated meta-analysis. Ultrasound Obstet Gynecol. 2017;50(3):302-314. doi:10.1002/uog.17484

16. Grace MR, Hardisty E, Dotters-Katz SK, Vora NL, Kuller JA. Cell-free DNA screening: Complexities and challenges of clinical implementation. Obstet Gynecol Surv. 2016;71(8):477-487. doi:10.1097/OGX.0000000000000342