Diagnostic Imaging Pathways - First Trimester Screening
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This pathway provides guidance on the screening of pregnant women to detect fetal abnormalities in early pregnancy.
Date reviewed: September 2018
Date of next review: September 2021
Published: April 2019
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Teaching Points
Teaching Points
- There are two screening tests available for the antenatal detection of Down Syndrome and other fetal aneuploidies: the combined first trimester screen and non-invasive prenatal testing (NIPT) with cell-free DNA (cfDNA)
- Combined first trimester screening identifies 86-93% of Down Syndrome cases at a false positive rate of 3-5% in studies using a risk cut-off of 1:300. The components of the test include:
- Age and past obstetric history
- Serum β-hCG and PAPP-A
- Fetal ultrasound scan with measurement of nuchal translucency thickness
- If the calculated risk is higher than 1:300 then further testing is recommended, either with secondary screening with cfDNA if available, or with invasive diagnostic testing (chorionic villus sampling or amniocentesis)
- NIPT is a maternal blood test that detects chromosomal abnormalities using cell-free fetal DNA in the maternal circulation. cfDNA has demonstrated high accuracy in the detection of common fetal autosomal trisomies (T21, T18 and T13) and has been clinically validated in both high-risk and general obstetric populations with a sensitivity >99% and false positive rate ≤0.1%
- If undertaken as an initial screening test, measurement of NT is not necessary with cfDNA but an ultrasound examination should be undertaken prior to confirm viability, gestational number and to assess for significant structural abnormalities
- cfDNA is not a diagnostic test, so a high-probability result requires confirmation with chorionic villus sampling or amniocentesis
- Increased NT thickness and low PAPP-A are risk factors for fetal structural anomalies, so further investigation may be warranted even in normal karyotype fetuses
counselling
Pre- and Post-test Counselling
- Counselling of patients for first trimester screening is usually performed by trained general practitioners, radiologists, obstetricians or clinical geneticists
- The objective of counselling is to inform patients about the benefits, risks and implications of screening to allow them to consent to the tests, understand the results and be suitably knowledgeable to make further decisions based on the results. All testing should be undertaken voluntarily 1
- The primary aim of screening is to identify women at increased risk of having babies with trisomy 21 (Down syndrome), trisomy 13 (Patau syndrome) and trisomy 18 (Edwards syndrome) and who may therefore benefit from further testing. Down syndrome is the most common and clinically significant aneuploidy. However, the process of scanning the fetus may, in some circumstances, reveal other morphological abnormalities such as a cystic hygroma (suggestive of Turner Syndrome), anencephaly, anterior abdominal wall and limb defects and other abnormalities which may or may not be related to other chromosomal abnormalities
- There are two available initial screening tests:
- Screening may involve combined first trimester screening only, cfDNA only or combined screening with cfDNA as a secondary screening test contingent on the result. 1-3 There are advantages and disadvantages to each screening strategy
combfts
Combined First Trimester Screening
- The combined first trimester screen calculates an adjusted risk for trisomies 21, 18 and 13 based on:
- Age and past obstetric history
- Maternal serum β-hCG and PAPP-A
- Fetal ultrasound scan with measurement of nuchal translucency thickness
- The blood test is optimal when taken from 9-13+6 weeks, and the ultrasound should be undertaken at 11 – 13+6 weeks gestation with chorionic villous sampling (CVS) available 4-6
- Test timing is optimal for PAPP-A at 9-10 weeks and β-hCG and ultrasound at 12 weeks 7
- The combination of fetal NT, maternal serum PAPP-A, β-hCG and maternal risk factors identifies 86-93% of Down Syndrome cases at a false positive rate of 3-5% in studies using a risk cut-off of 1:300 7-14
- Women with a risk between 1:100 and 1:300 are further counselled on additional testing with chorionic villus sampling (CVS), amniocentesis or cell-free DNA (cfDNA) if available. There are advantages and disadvantages to each diagnostic strategy
- Current expert opinion suggests that cfDNA should not replace invasive diagnostic testing in women with very high risk (>1:10) after combined screening, 2 although there are no trials specifically assessing the ideal cut-off. Australian guidelines recommend invasive testing in women with cFTS risk >1:100, as there is an 18% chance of any major chromosomal abnormality, including a 3% risk of an abnormality not detectable on cfDNA testing 1,15
- Diagnostic testing is also recommended if any significant structural abnormality is detected 1,3
- Women with intermediate risk between 1:300 and 1:1000 may be offered cfDNA as a secondary screening test, or no further testing
- cfDNA is a more sensitive test than combined screening so may provide further reassurance
- Any woman who is not sufficiently reassured by her aneuploidy probability from prior cFTS can be offered either follow-up screening with cfDNA or invasive tesing, considering the advantages and disadvantages of each 3
- Current guidelines recommend further testing in increased NT thickness ≥3.5mm, regardless of cFTS result 3
- cfDNA may be reassuring in a slightly increased NT, however diagnostic testing is more appropriate if NT is grossly abnormal. Read more about the management of increased nuchal translucency thickness
- Diagnostic testing is also recommended if PAPP-A or β-hCG are less than 0.2 multiples of the median (MoM) as there is a 5% chance of an atypical chromosomal abnormality not detectable on cfDNA 3,15
cfdna
Cell-free DNA
- Recently, screening with cfDNA in the maternal circulation has demonstrated high accuracy in the detection of common fetal autosomal trisomies (T21, T18 and T13) and has been clinically validated in both high-risk and general obstetric populations with a sensitivity >99% and false positive rate ≤0.1% 16-22
- Diagnostic testing with chorionic villus sampling (CVS), amniocentesis is required to confirm a high-probability cfDNA result before deciding how to manage the pregnancy, as there is a significant false positive rate
- cfDNA may be offered to women as an initial screening test or as a secondary screening test contingent on the combined first trimester screen result
- Australian guidelines recommend that cfDNA should not replace invasive diagnostic testing in women with very high risk cFTS (>1:100, as there is an 18% chance of any major chromosomal abnormality, including a 3% risk of an abnormality not detectable on cfDNA testing) 1,15
- cfDNA may be offered to women with a high risk combined screening result 1:100 to 1:300 or with an intermediate risk 1:300 to 1:1000
- The optimum thresholds for high and intermediate risk groups may vary according to local factors 2,23
- cfDNA does not replace first trimester ultrasound. 2 Women electing to undergo cfDNA as a first-line screening test should undergo a first trimester ultrasound scan to confirm gestational age, number and that there are no major structural abnormalities that would warrant diagnostic testing
- Subsequently detected significant structural abnormalities warrant invasive diagnostic testing even if cfDNA has been low-probability 2,3
- Women with a high-probability cfDNA result should be counselled for invasive diagnostic testing
- There is a higher rate of aneuploidy in women with failed tests, so proceeding to invasive diagnostic testing is recommended 3, 24,25
- The main advantage of cfDNA is that invasive testing and its associated risks may be avoided. Current literature suggests that cfDNA decreases the number of invasive procedures without reducing the detection rate of aneuploidies when used as a second-line test, 26-28 however a recent study did not find any reduction in the rate of miscarriage 29
- There is less information on the use cfDNA in twin pregnancies, however evidence that suggests that screening for T21 with cfDNA is feasible. 21,30-33 Unlike cFTS, cfDNA does not determine individual probabilities for each fetus 3
- The main limitations of cfDNA are the high cost to the patient and the time delay to results (and further diagnostic testing if needed), which could affect decision-making about continuing the pregnancy. The cost and time to result are both continuing to decrease as cfDNA becomes more widely available
us
Ultrasound
- Regardless of whether women elect for combined first trimester screening or cfDNA, the purpose of the first trimester ultrasound is to:
- Determine fetal viability
- Detect multiple pregnancies
- Date the pregnancy
- Identify major anatomical defects
- If combined first trimester screening is undertaken, measurement of nuchal translucency (NT) is required for risk calculation
- Nuchal translucency is the normal clear area in the fetal neck that lies between the skin and the soft tissues overlying the cervical spine, on a sagittal section through the fetus
- The first trimester screening ultrasound should be performed between 11 weeks and 13 weeks 6 days gestation when crown-rump length (45-84mm) is optimal and timely diagnostic testing with chorionic villous sampling (CVS) is available. 4-6 It can only be performed with the appropriate software endorsed by the Fetal Medicine Foundation by trained ultrasound operators to achieve uniform results
- Increasing NT thickness is associated with an increasing rate of chromosomal defects and structural abnormalities. 6,34,35 A value ≥ 95th percentile (~2.1-2.7mm depending on gestational age) is generally considered abnormal, but adverse outcomes increase exponentially after ≥3.5mm, equating to the 99th percentile
- Currently the significance of increased NT in women who have had low-probability cfDNA is uncertain. 2,36 An ultrasound examination is recommended prior to cfDNA in the first trimester. 2 This allows for detection of structural abnormalities that may warrant diagnostic testing 3
- Soft markers for T21 should not be assessed in women with low-probability cfDNA 2,3
- Increased NT thickness is also associated with structural abnormalities and adverse outcomes in karyotypically normal fetuses, especially cardiac malformations. 37-41 Patients with NT measures of ≥3.5mm, equating to ≥99th percentile or ≥2.5 MoM for gestational age, 42 should be referred for specialist, targeted early ultrasound at 16 weeks, fetal echocardiography, or both, even if the karyotype is normal or cfDNA is low-probability 43
- First trimester ductus venosus screening for congenital heart disease is only 83% and 80% sensitive and specific in patients with increased NT, and 15% and 96% specific in patients with normal NT 44
- A routine mid-trimester ultrasound has been shown to improve the prenatal detection of major fetal abnormalities. 45 Ultrasound examination is recommended between 18-22 weeks as visualisation of fetal structural anomalies is optimised at this time 46,47
- Assessment of neural tube defects (eg, spina bifida) is usually performed during the 19 week fetal morphology ultrasound scan and this cannot be easily detected in the first trimester screen
Screening for aneuploidy
Detecting other abnormalities or adverse perinatal outcomes
cvsamnio
Chorionic Villus Sampling and Amniocentesis
- Chorionic villus sampling (CVS) and amniocentesis are invasive diagnostic tests for chromosomal abnormalities
- Both are performed under ultrasound guidance and may be performed by a trained radiologist or obstetrician
- Invasive tests are associated with risks, most notably the risk of pregnancy loss, however current risks are likely much lower than previously quoted
- Can be performed earlier than amniocentesis; CVS may be perfomed in the first trimester from 11 weeks 23
- Chorionic villi are collected for genetic evaluation without entering the amniotic sac, by a transabdominal or transvaginal approach 48
- Advantage: earlier prenatal diagnosis may give the option for an earlier (and therefore safer) termination if desired 48
- Disadvantage: 1-2% results may be false positives due to confined placental mosaicism 49
- Recent studies report the procedure-related risk of fetal loss as approximately 1 in 287 or lower 50-52
- May be perfomed in the second trimester from 15 weeks 23
- A sterile needle is introduced into the amniotic sac under ultrasound guidance and amniotic fluid is obtained and sent for testing 48
- Risk of pregnancy loss previously reported as lower than CVS, but recent studies have reported similar risk estimates 50-52
Chorionic Villus Sampling (CVS)
Amniocentesis
nuchal
Nuchal Translucency
- Nuchal translucency is the normal clear area in the fetal neck that lies between the skin and the soft tissues overlying the cervical spine, on a sagittal section through the fetus
- Increasing NT thickness is associated with an increasing rate of chromosomal defects and structural abnormalities. 6,34,35 A value ≥ 95th percentile (~2.1-2.7mm depending on gestational age) is generally considered abnormal, but adverse outcomes increase exponentially after ≥3.5mm, equating to the 99th percentile
- Current guidelines recommend further testing for NT ≥3.5mm regardless of cFTS result 3
- cfDNA and invasive testing are both options to investigate increased NT, however invasive testing is generally more appropriate if NT is grossly abnormal. Pregnancies with increased NT should be referred and the decision for further testing usually requires specialist input
- It is important to note that 86% of chromosomally normal pregnancies with NT<4.5mm result in healthy live births. 37 If chromosomally normal fetuses with increased NT have a normal ultrasound by 20 weeks gestation, there is a 96% chance of good outcome 53
- Patients with NT ≥3.5mm should be referred for specialist, targeted early ultrasound at 16 weeks, fetal echocardiography, or both, even if the karyotype is normal or cfDNA is low-probability 43
- Increased NT thickness is associated with structural abnormalities and adverse outcomes, even in karyotypically normal fetuses, especially cardiac malformations 37-41
- Although serious, the incidence of cardiac defects is low. A meta-analysis of screening studies reported fetal echocardiography in all chromosomally normal fetuses with NT ≥99th percentile would identify only one major cardiac defect for every 16 patients examined 40
References
References
Date of literature search: August 2018
The search methodology is available on request. Email
References are graded from Level I to V according to the Oxford Centre for Evidence-Based Medicine, Levels of Evidence. Download the document
- Gabbett M, Halliday J, Hyett J, White S, Hui L, McGillivray G. Prenatal assessment of fetal structural conditions. Australia: Royal Australian and New Zealand College of Obstetricians and Gynaecologists; 2018. (Guideline). View the reference
- Salomon LJ, Alfirevic Z, Audibert F, Kagan KO, Paladini D, Yeo G, et al. ISUOG updated consensus statement on the impact of cfDNA aneuploidy testing on screening policies and prenatal ultrasound practice. Ultrasound Obstet Gynecol. 2017;49(6):815-6. (Guideline). View the reference
- Rieder W, White S, McGillivray G, Hui L. Contemporary prenatal aneuploidy screening practice in Australia: Frequently asked questions in the cell-free DNA era. Aust N Z J Obstet Gynaecol. 2018;58(4):397-403. (Guideline). View the reference
- Whitlow BJ, Economides DL. The optimal gestational age to examine fetal anatomy and measure nuchal translucency in the first trimester. Ultrasound Obstet Gynecol. 1998;11(4):258-61. (Level II evidence). View the reference
- Mulvey S, Baker L, Edwards A, Oldham J, Shekleton P, Wallace EM. Optimising the timing for nuchal translucency measurement. Prenat Diagn. 2002;22(9):775-7. (Level II evidence). View the reference
- Nicolaides KH. Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities. Am J Obstet Gynecol. 2004;191(1):45-67. (Review article). View the reference
- Wright D, Spencer K, Kagan KK, Torring N, Petersen OB, Christou A, et al. First-trimester combined screening for trisomy 21 at 7-14 weeks' gestation. Ultrasound Obstet Gynecol. 2010;36(4):404-11. (Level II evidence). View the reference
- Malone FD, Canick JA, Ball RH, Nyberg DA, Comstock CH, Bukowski R, et al. First-trimester or second-trimester screening, or both, for Down's syndrome. N Engl J Med. 2005;353(19):2001-11. (Level II evidence). View the reference
- Spencer K, Spencer CE, Power M, Dawson C, Nicolaides KH. Screening for chromosomal abnormalities in the first trimester using ultrasound and maternal serum biochemistry in a one-stop clinic: a review of three years prospective experience. BJOG. 2003;110(3):281-6. (Level II evidence). View the reference
- Hadlow NC, Hewitt BG, Dickinson JE, Jacoby P, Bower C. Community-based screening for Down's Syndrome in the first trimester using ultrasound and maternal serum biochemistry. BJOG. 2005;112(11):1561-4. (Level II-III evidence). View the reference
- Ekelund CK, Jørgensen FS, Petersen OB, Sundberg K, Tabor A. Impact of a new national screening policy for Down’s syndrome in Denmark: population based cohort study. BMJ. 2008;337 (Level II-III evidence). View the reference
- Bindra R, Heath V, Liao A, Spencer K, Nicolaides KH. One-stop clinic for assessment of risk for trisomy 21 at 11-14 weeks: a prospective study of 15 030 pregnancies. Ultrasound Obstet Gynecol. 2002;20(3):219-25. (Level II evidence). View the reference
- Maxwell S, Brameld K, Bower C, Dickinson JE, Goldblatt J, Hadlow N, et al. Socio-demographic disparities in the uptake of prenatal screening and diagnosis in Western Australia. Aust N Z J Obstet Gynaecol. 2011;51(1):9-16. (Level II evidence). View the reference
- Alldred SK, Takwoingi Y, Guo B, Pennant M, Deeks JJ, Neilson JP, et al. First trimester ultrasound tests alone or in combination with first trimester serum tests for Down's syndrome screening. Cochrane Database Syst Rev. 2017;3:Cd012600. (Level I evidence). View the reference
- Lindquist A, Poulton A, Halliday J, Hui L. Prenatal diagnostic testing and atypical chromosome abnormalities following combined first-trimester screening: implications for contingent models of non-invasive prenatal testing. Ultrasound Obstet Gynecol. 2018;51(4):487-92. (Level II evidence). View the reference
- Nicolaides KH, Syngelaki A, Ashoor G, Birdir C, Touzet G. Noninvasive prenatal testing for fetal trisomies in a routinely screened first-trimester population. Am J Obstet Gynecol. 2012;207(5):374.e1-6. (Level II evidence).
- Dan S, Wang W, Ren J, Li Y, Hu H, Xu Z, et al. Clinical application of massively parallel sequencing-based prenatal noninvasive fetal trisomy test for trisomies 21 and 18 in 11,105 pregnancies with mixed risk factors. Prenat Diagn. 2012;32(13):1225-32. (Level II evidence). View the reference
- Badeau M, Lindsay C, Blais J, Nshimyumukiza L, Takwoingi Y, Langlois S, et al. Genomics-based non-invasive prenatal testing for detection of fetal chromosomal aneuploidy in pregnant women. Cochrane Database Syst Rev. 2017;11:Cd011767. (Level I evidence). View the reference
- Norton ME, Brar H, Weiss J, Karimi A, Laurent LC, Caughey AB, et al. Non-Invasive Chromosomal Evaluation (NICE) Study: results of a multicenter prospective cohort study for detection of fetal trisomy 21 and trisomy 18. Am J Obstet Gynecol. 2012;207(2):137.e1-8. (Level II evidence). View the reference
- 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):302-14. (Level I evidence). View the reference
- Zhang H, Gao Y, Jiang F, Fu M, Yuan Y, Guo Y, et al. Non-invasive prenatal testing for trisomies 21, 18 and 13: clinical experience from 146,958 pregnancies. Ultrasound Obstet Gynecol. 2015;45(5):530-8. (Level II evidence). View the reference
- Iwarsson E, Jacobsson B, Dagerhamn J, Davidson T, Bernabe E, Heibert Arnlind M. Analysis of cell-free fetal DNA in maternal blood for detection of trisomy 21, 18 and 13 in a general pregnant population and in a high risk population - a systematic review and meta-analysis. Acta Obstet Gynecol Scand. 2017;96(1):7-18. (Level I evidence). View the reference
- McGillivray G, Hui L, Halliday J. Prenatal screening and diagnosis of chromosomal and genetic conditions in the fetus in pregnancy. Australia: Royal Australian and New Zealand College of Obstetricians and Gynaecologists; 2016. (Guideline). View the reference
- Revello R, Sarno L, Ispas A, Akolekar R, Nicolaides KH. Screening for trisomies by cell-free DNA testing of maternal blood: consequences of a failed result. Ultrasound Obstet Gynecol. 2016;47(6):698-704. (Level II evidence). View the reference
- Chan N, Smet ME, Sandow R, da Silva Costa F, McLennan A. Implications of failure to achieve a result from prenatal maternal serum cell-free DNA testing: a historical cohort study. BJOG. 2018;125(7):848-55. (Level II evidence). View the reference
- Martinez-Payo C, Bada-Bosch I, Martinez-Moya M, Perez-Medina T. Clinical results after the implementation of cell-free fetal DNA detection in maternal plasma. J Obstet Gynaecol Res. 2018 (Level II evidence). View the reference
- Hui L, Norton M. What is the real "price" of more prenatal screening and fewer diagnostic procedures? Costs and trade-offs in the genomic era. Prenat Diagn. 2018;38(4):246-9. (Level III evidence). View the reference
- Oepkes D, Page-Christiaens GC, Bax CJ, Bekker MN, Bilardo CM, Boon EMJ, et al. Trial by Dutch laboratories for evaluation of non‐invasive prenatal testing. Part I—clinical impact. Prenat Diagn. 2016;36(12):1083-90. (Level II evidence). View the reference
- Malan V, Bussières L, Winer N, et al. Effect of cell-free DNA screening vs direct invasive diagnosis on miscarriage rates in women with pregnancies at high risk of trisomy 21: A randomized clinical trial. JAMA. 2018;320(6):557-65. (Level II evidence). View the reference
- Liao H, Liu S, Wang H. Performance of non-invasive prenatal screening for fetal aneuploidy in twin pregnancies: a meta-analysis. Prenat Diagn. 2017;37(9):874-82. (Level I evidence). View the reference
- Fosler L, Winters P, Jones KW, Curnow KJ, Sehnert AJ, Bhatt S, et al. Aneuploidy screening by non-invasive prenatal testing in twin pregnancy. Ultrasound Obstet Gynecol. 2017;49(4):470-7. (Level II evidence). View the reference
- Huang X, Zheng J, Chen M, Zhao Y, Zhang C, Liu L, et al. Noninvasive prenatal testing of trisomies 21 and 18 by massively parallel sequencing of maternal plasma DNA in twin pregnancies. Prenat Diagn. 2014;34(4):335-40. (Level II evidence). View the reference
- Bevilacqua E, Gil MM, Nicolaides KH, Ordonez E, Cirigliano V, Dierickx H, et al. Performance of screening for aneuploidies by cell-free DNA analysis of maternal blood in twin pregnancies. Ultrasound Obstet Gynecol. 2015;45(1):61-6. (Level II evidence). View the reference
- Kagan KO, Avgidou K, Molina FS, Gajewska K, Nicolaides KH. Relation between increased fetal nuchal translucency thickness and chromosomal defects. Obstet Gynecol. 2006;107(1):6-10. (Level II-III evidence). View the reference
- Snijders RJ, Noble P, Sebire N, Souka A, Nicolaides KH. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal-translucency thickness at 10-14 weeks of gestation. Fetal Medicine Foundation First Trimester Screening Group. Lancet. 1998;352(9125):343-6. (Level II evidence). View the reference
- Huang LY, Pan M, Han J, Zhen L, Yang X, Li DZ. What would be missed in the first trimester if nuchal translucency measurement is replaced by cell free DNA foetal aneuploidy screening? J Obstet Gynaecol. 2018:1-4. (Level III evidence). View the reference
- Souka AP, Krampl E, Bakalis S, Heath V, Nicolaides KH. Outcome of pregnancy in chromosomally normal fetuses with increased nuchal translucency in the first trimester. Ultrasound Obstet Gynecol. 2001;18(1):9-17. (Level III evidence). View the reference
- Westin M, Saltvedt S, Bergman G, Almstrom H, Grunewald C, Valentin L. Is measurement of nuchal translucency thickness a useful screening tool for heart defects? A study of 16,383 fetuses. Ultrasound Obstet Gynecol. 2006;27(6):632-9. (Level II evidence). View the reference
- Bahado-Singh RO, Wapner R, Thom E, Zachary J, Platt L, Mahoney MJ, et al. Elevated first-trimester nuchal translucency increases the risk of congenital heart defects. Am J Obstet Gynecol. 2005;192(5):1357-61. (Level II evidence). View the reference
- Makrydimas G, Sotiriadis A, Ioannidis JP. Screening performance of first-trimester nuchal translucency for major cardiac defects: a meta-analysis. Am J Obstet Gynecol. 2003;189(5):1330-5. (Level I evidence). View the reference
- Nicolaides KH, Heath V, Cicero S. Increased fetal nuchal translucency at 11-14 weeks. Prenat Diagn. 2002;22(4):308-15. (Level III evidence). View the reference
- Simpson LL, Malone FD, Bianchi DW, Ball RH, Nyberg DA, Comstock CH, et al. Nuchal translucency and the risk of congenital heart disease. Obstet Gynecol. 2007;109(2 Pt 1):376-83. (Level II evidence). View the reference
- American College of Obstetricians and Gynecologists. Practice Bulletin No. 163: Screening for Fetal Aneuploidy. Obstet Gynecol. 2016;127(5):e123-37. (Guideline). View the reference
- Papatheodorou SI, Evangelou E, Makrydimas G, Ioannidis JP. First-trimester ductus venosus screening for cardiac defects: a meta-analysis. BJOG. 2011;118(12):1438-45. (Level I evidence). View the reference
- Whitworth M, Bricker L, Mullan C. Ultrasound for fetal assessment in early pregnancy. Cochrane Database Syst Rev. 2015(7):Cd007058. (Level I evidence). View the reference
- McLennan A, Walker S. Prenatal assessment of fetal structural conditions. Australia: Royal Australian and New Zealand College of Obstetricians and Gynaecologists; 2016. (Guideline). View the reference
- Edwards L, Hui L. First and second trimester screening for fetal structural anomalies. Semin Fetal Neonatal Med. 2018;23(2):102-11. (Review article). View the reference
- Carlson LM, Vora NL. Prenatal diagnosis: screening and diagnostic tools. Obstet Gynecol Clin North Am. 2017;44(2):245-56. (Review article). View the reference
- Baffero GM, Somigliana E, Crovetto F, Paffoni A, Persico N, Guerneri S, et al. Confined placental mosaicism at chorionic villous sampling: risk factors and pregnancy outcome. Prenat Diagn. 2012;32(11):1102-8. (Level II-III evidence). View the reference
- Akolekar R, Beta J, Picciarelli G, Ogilvie C, D'Antonio F. Procedure-related risk of miscarriage following amniocentesis and chorionic villus sampling: a systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2015;45(1):16-26. (Level I evidence). View the reference
- Beta J, Lesmes-Heredia C, Bedetti C, Akolekar R. Risk of miscarriage following amniocentesis and chorionic villus sampling: a systematic review of the literature. Minerva Ginecol. 2018;70(2):215-9. (Level I evidence). View the reference
- Wulff CB, Gerds TA, Rode L, Ekelund CK, Petersen OB, Tabor A. Risk of fetal loss associated with invasive testing following combined first-trimester screening for Down syndrome: a national cohort of 147,987 singleton pregnancies. Ultrasound Obstet Gynecol. 2016;47(1):38-44. (Level II evidence). View the reference
- Bilardo CM, Muller MA, Pajkrt E, Clur SA, van Zalen MM, Bijlsma EK. Increased nuchal translucency thickness and normal karyotype: time for parental reassurance. Ultrasound Obstet Gynecol. 2007;30(1):11-8. (Level III evidence). View the reference
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