The use of ultrasound in obstetrics has become

Transcription

1 First Trimester Antenatal Ultrasonographic Screening for Aneuploidy - So Far So Good Hetal Parikh*, Sameer Raniga**, Pankaj Desai, Abhishek Arora Abstract Ultrasound has become an integral component of obstetric care, with the vast majority of patients having at least one ultrasound examination during pregnancy. Recent advances in obstetric ultrasonography (USG) have increased its importance in managing pregnancies at risk for aneuploidy and structural abnormality. Antenatal screening of aneuploidy, particularly Down syndrome includes biochemical markers, USG and invasive tests like chorionic villus sampling (CVS) and amniocentesis. In this article, we review and compare these screening modalities with emphasis on USG. In light of current literature, we will discuss first trimester sonographic markers associated with aneuploidy viz. nuchal translucency (NT), fetal nasal bone and ductus venosus flow and their statistical as well as clinical significance in the detection of aneuploidy. We will review the current data and status of first trimester screening for aneuploidy. Keywords: Aneuploidy, prenatal ultrasound, first trimester, nuchal translucency, fetal nasal bone, ductus venosus flow The use of ultrasound in obstetrics has become ubiquitous. Ultrasound has become an integral component of obstetric care, with the vast majority of patients undergoing at least one ultrasound examination during pregnancy. Recent advances in obstetric ultrasonography have increased its importance in managing pregnancies at risk for aneuploidy and structural abnormality. Life of an obstetric ultrasonologist was never so difficult and so easy at the same time. Life is easy because of technical developments and types of sophisticated ultrasound units available with 3D and 4D facilities and high-resolution images. With these advancements gross malformations are easy to detect. Life is difficult because of the whole new crop of ultrasound findings with uncertain clinical significance that can be seen with these units. With newer sophisticated sonography units, more and more subtle markers are being picked up, the clinical relevance and importance of these markers is *Specialist Dept. of Obstetrics and Gynecology, Armed Forces Hospital, Muscat, Oman **Consultant Radiologist Khoula Hospital, Muscat, Oman Associate Professor Dept. of Obstetric and Gynecology, SSG Hospital and Medical College, Baroda Consultant Radiologist Tata Memorial Hospital, Mumbai Address for correspondence Dr Hetal Parikh 81, Shantinagar, Tarsali Road, Vadodara samhet10200@yahoo.com; sameerraniga@yahoo.com not still fully understood. The most important decision of advising the invasive tests for the confirmation of the suspected aneuploidy in these patients with soft markers is a challenge. Antenatal screening of aneuploidy, particularly Down syndrome includes biochemical markers, ultrasonography (USG) and invasive tests like chorionic villus sampling (CVS) and amniocentesis. At present, these invasive procedures are considered as gold standard for the diagnosis of chromosomal anomalies or other genetic diseases. However, these procedures are associated with a finite risk of morbidity and mortality to the fetus. The risk associated with these tests, availability and the cost of analysis precludes the adoption of methods for mass screening of pregnant women. Therefore, diagnostic procedures are only offered on a limited basis to high-risk pregnancies where the benefit outweighs the risk. In low-risk pregnancies, where there is low likelihood of diagnosing a chromosomal abnormality, prenatal diagnosis generally consists of screening procedures by means of ultrasound and maternal serum biochemistry. The incidence of Down syndrome increases with maternal age. The levels of four maternal serum biochemical markers in the second trimester of pregnancy - alpha-fetoprotein (AFP), human chorionic gonadotropin (hcg), estriol and inhibin A - have been found to be associated with Down syndrome. 1,2 Second trimester maternal biochemical serum screening for 550 Indian Journal of Clinical Practice, Vol. 22, No. 11, April 2012

2 abnormal levels of some or all of these markers is now part of routine obstetrical care in the United States and allows the detection of approximately 60% of cases of Down syndrome, with a false-positive rate of 7%. 2,3 Since, there is considerable overlap of maternal serum values for unaffected and chromosomally abnormal fetuses, a positive screening test result needs to be followed by a diagnostic test. Amniocentesis can reliably determine fetal karyotype, but there is a % fetal mortality rate associated with this procedure. 4 Ultrasound is the imaging modality for the prenatal screening programs aimed at identifying fetal chromosomal abnormalities. Its safety during pregnancy and noninvasiveness are two of its most desirable traits. We will review the role of USG in the first (up to 13 weeks and 6 days) trimester screening for aneuploidy. The most common reason for prenatal diagnosis of chromosome abnormalities is to look for evidence of trisomies, Turner syndrome and triploidy. Trisomy 13, 18, and 21 are the most common, with trisomy 21 comprising about half of all the trisomies identified. In the first trimester, a common feature of many chromosomal defects is increased nuchal translucency (NT) thickness. In later pregnancy, each chromosomal defect has its own syndromal pattern of abnormalities. We also review and compare these screening modalities with emphasis on USG. In light of current literature, we will discuss first trimester sonographic markers associated with aneuploidy and its statistical as well as clinical significance in the detection of aneuploidy. FIRST TRIMESTER SCREENING FOR ANEUPLOIDY Prenatal screening for aneuploidies and especially Down syndrome has expanded substantially over the past 20 years. Initially, only women at risk were offered the option of invasive prenatal diagnosis. However, with the advent of second trimester multiple marker serum screening test or abnormal second trimester sonographic markers or soft signs, prenatal invasive tests are offered to general obstetric population identified to have one of these markers positive. The most efficient multiple marker screening test in second trimester is known as the quad screen, a biochemical marker panel comprised of AFP, hcg, unconjugated estriol and inhibin-a. 5 This combination approach is far from perfect and has sensitivity of 67-76% for Down syndrome with a 5% false-positive rate. 6 This common method of screening has several limitations, the earliest it can reliably be performed is 15 weeks and hence limiting the choice of definitive diagnosis of aneuploidy to amniocentesis. Over 25% of Down syndrome cases cannot be detected with this screening approach and with the false-positive rate of 5% and the pregnancy loss rate of one in 200 associated with amniocentesis, about one normal fetus is lost for every three fetuses with Down syndrome detected. 7 It is easy to understand that the current approach of second trimester screening is far from perfect and hence a great deal of interest has been directed toward shifting prenatal screening for aneuploidies to the first trimester using sonographic measurement of the fetal NT alone and in combination with other sonographic and biochemical markers. FETAL NUCHAL TRANSLUCENCY In 1992, Nicolaides et al introduced the term nuchal translucency, which was defined as the thickness of the translucent space between the skin and the soft tissue overlying the fetus cervical spine, measured in millimeters and tenths of a millimeter via ultrasound. 8 NT ultrasound has pushed prenatal screening for Down syndrome into the first trimester. NT refers to the normal subcutaneous fluid-filled space between the back of the fetal neck and the overlying skin. It can be measured accurately and reproducibly on ultrasound between 10 and 14 weeks of gestation (Fig. 1). It is seen that thickening of NT is associated with increased risk for Down syndrome, other aneuploidies, major structural malformations and adverse pregnancy outcomes (Fig. 2). The association increases with increase in nuchal thickness. The etiology for this nuchal fluid accumulation has still not been defined and various theories offer an explanation. The most cited are: Deficient and transitory lymphatic drainage of the cervical region due to disorders in the lymphatic connections, 9,10 excessive perfusion of the protective mechanism of the central nervous system as a result of the rapid growth of the initial placenta which consequently increases the circulatory volume 11 and cardiac alterations - mainly the narrowing of the aortic isthmus and consequently increasing the vascular flow of the fetal cervical region or cardiac failure with abnormal ductus venosus tracing. 15 Some publications consider NT 2.5 mm 10,16,17 as positive screening values, whereas the great majority uses a fixed value of NT 3 mm However, recent literature suggests that it is inappropriate to choose a single millimeter cut-off to define a specific NT measurement as abnormal. 22 They recommended the use of 95th percentile value for the specified gestational age or multiples of the median as the cut-off upper limit for the diagnosing thickened NT. The natural increase of NT Indian Journal of Clinical Practice, Vol. 22, No. 11, April

3 Figure 1. Shows the correct technique for measuring NT in first trimester. Figure 2. Shows thickening of NT. measurement by 17% per week should be considered when calculating cut-offs for the use with an increased NT. 23 Unfortunately, detailed information on such cutoffs is not available in the literature. Dr Ellen Mozurkewich and coauthors of the University of Michigan, Ann Arbor analyzed 27 studies at the annual meeting of Society for Maternal-Fetal medicine, which included 1,63,450 high-risk subjects and reported that NT measurements identified 78% of fetuses affected by trisomy 21, 82% of those with trisomy 18, 90% of those with trisomy 13 and 96% of those with monosomy X. The negative predictive value for all four conditions was 99%. The false-positive rate for each of the conditions was <5%. They concluded that NT is a sensitive marker for identifying aneuploidy in highrisk population. 24 Few studies have evaluated the NT thickness and prevalence of aneuploidy and suggested that the association between the two becomes stronger with increasing thickness of NT. 18,25 Pandya et al found prevalence of aneuploidy 7% at 3 mm NT and 70-78% at 8 and 9 mm thickness, respectively. 25 NT shows spontaneous regression by 20 weeks. The current literature suggests that NT ultrasound screening has tremendous potential as powerful prenatal screening for aneuploidy. Results of studies in the general obstetric population in a routine clinical setting have been mixed, with a range of detection rates for Down syndrome between 29 and 100%. Brigatti et al reviewed 30 published studies on the performance of NT-based screening for Down syndrome in the general population between 1966 and April They used data from all 30 studies, a total 3,16,311 patients and concluded an overall sensitivity for Down syndrome of 77% with a 6% false-positive rate. An abnormal NT measurement is 13 times more likely to be present in cases of Down syndrome. Because Down syndrome pregnancies are more likely to result in fetal demise in first trimester as many as 40% - the figure mentioned above may be overestimated. They also mentioned the shortcoming of current literature on NT- in form of consideration of first trimester loss of Down fetuses, karyotypic confirmation of all suspected cases, lack of information on the success rate at obtaining an NT measurement and lack of control group for comparison between first trimester screening and the current standard of care of second trimester multiple marker screening. 7 NT measurement can also detect other aneuploidies and the detection rate for trisomy 18 was 81% and Turner syndrome was 80% and 63% for triploidy. 22 Approximately 80% of the affected fetuses with these aneuploidies result in spontaneous abortion. 26 NT-based screening may preferentially identify those pregnancies with the highest likelihood of intrauterine death. 27 Hence, the role of first trimester screening is debatable. NT Measurement with Maternal Serum Markers in First Trimester: Combined and Integrated Approach First trimester maternal serum markers include free b-component of hcg (FβhCG) and PAPP-A. The combination of increased level of FβhCG and reduced level of PAPP-A with maternal age demonstrate a detection rate of approximately 60% with a 5% falsepositive rate. 28 Brigatti et al reviewed the combined approach by maternal serum markers and NT-based screening Indian Journal of Clinical Practice, Vol. 22, No. 11, April 2012

4 As mentioned in their paper, the first trimester serum markers seem to be independent of NT. They reviewed seven published studies of the combined method of screening and found the overall sensitivity for Down syndrome of 82% for a 5% false-positive rate. Using the NT measurement and serum markers from the first trimester in combination with maternal serum analytes from the second trimester will provide one single Down syndrome risk assessment and is found to be superior to either approach separately in each trimester alone. This two-step approach, commonly known as the integrated test, involves the combination of NT ultrasound and maternal serum PAPP-A in the first trimester followed by maternal serum AFP, hcg, unconjugated estriol and inhibin-a in the second trimester, with a single result proved in the second trimester. This integrated test has a higher sensitivity with less false-positive rates. First trimester screening does not eliminate the need for second trimester ultrasound for the detection of gross structural fetal anomalies. NT ultrasound is extremely operator-dependent. Quality control is the most important factor to standardize the practice of NT ultrasound. Criteria to maximize good quality of NT ultrasound includes: Gestation should be limited between 10 and 14 weeks (Crown Rump Length [CRL] mm), fetus should be examined in a mid-sagittal plane, fetal neck should be in a neutral position, fetal images should occupy at least 75% of the viewable screen, fetal movement should be awaited to distinguish between amnion and overlying fetal skin, calipers should be placed perpendicular to the fetal body axis, at least three NT measurements should be obtained, with the mean value and at least 20 minutes may need to be dedicated to the NT measurement before abandoning the effort as failed. 7 First Trimester Fetal Nasal Bone Ossification of the nasal bones first appears at a CRL of 42mm, and nasal bone increases linearly with gestation. Fetuses with Down syndrome have a flat face with small nasal bone. In a study that evaluated the association between nasal bone hypoplasia and Down syndrome at weeks gestation, nasal ossification was absent in 73% of Down syndrome fetuses versus 0.5% of chromosomally normal fetuses. 29 Few other studies concluded a similar association between absence of fetal nasal bone at weeks sonography and its association with Down syndrome The authors of the above study believed that the absence of fetal nasal bone to be independent of NT size and the two ultrasound screening methods could be combined into one modality, with a predicted sensitivity of 85% for a 1% false-positive rate. The adequate imaging of the fetal nose can be obtained in a midsagittal plane of the fetus, in perfect profile and with slight neck flexion; the fetal spine should be facing downward and two echogenic lines at the fetal nose bone profile the superficial one of the nasal skin and the deeper echogenic line representing the nasal bone. 7 First Trimester Fetal Ductus Venosus Flow First trimester ductus venosus (DV) flow studies have been identified as useful for aneuploidy screening. Forward biphasic pulsatile flow is normal, whereas reversed flow at the time of atrial contraction has been associated with aneuploidy and cardiac defects. Abnormality in the ductus flow represents the presence of complex cardiac anomalies, which may or may not be detected early in the antenatal period. DV flow velocimetry following an NT ultrasound evaluation is useful for modifying the risk for aneuploidy; it increases the detection rate and reduces the falsepositive rate. Accurate measurement of DV flow is very important and technically challenging as the DV is as small as 2 mm at weeks. Quality control and technical competence with experienced sonographer is required. It is desirable to use the smallest possible Doppler sample volume, keep the sample midway between the umbilical venous sinus and the inferior vena cava. 7 Mavrides et al showed a clear association between abnormal flow in the DV and fetal aneuploidy. They concluded that the use of DV velocimetry in combination with NT is better than either test alone, since it increases the sensitivity in the detection of Down syndrome to 94% and decreases the likelihood ratio of a negative test to Borrell et al concluded in their study that there is a high proportion of fetuses with trisomies 21, 18 and 13 (around 75%) in which the DV PIV (Pulsatility index for veins) is increased (above the 95th percentile) at weeks and this proportion is similar to that observed for increased NT measurement. 39 Other Sonographic Signs in First Trimester Other sonographic signs in the first trimester that suggest the presence of aneuploidy includes sacembryo disproportion: Too small gestational sac, too large gestational sac, amnion too close to embryo, large, small or irregular yolk sac, shapeless embryo However, these signs are not of clinical significance, as most of these embryos will end up in spontaneous abortion. Trisomy is one of the commonest aneuploidy in these fetuses. 41 Indian Journal of Clinical Practice, Vol. 22, No. 11, April

5 CURRENT STATUS OF SCREENING ULTRASOUND IN FIRST TRIMESTER Ultrasound is an excellent tool, but it is far from perfect. If patients are falsely reassured by an ultrasound examination, they may decide to forego definitive testing when that is indeed what they desire. Equally worrisome is the patient who is counseled that a particular finding has more significance than it does, and she decides to have an invasive test that is not indicated and not really desired. In the current intense medicolegal environment, often medical recommendations are made more to protect the providers from possible litigation than they are based on true medical opinion. Clearly, an invasive test is unlikely to miss the diagnosis. 7 It is a time for introspection. What is the current status of USG and sonographically detected markers in our country? First of all, no definite large-scale prospective or retrospective study is being done in our country to have our own norms and hence we will have to rely on the international statistics and literature available. Most of our patients fall in the category of low risk if we consider age 35 or above as the high-risk patients. The significance of these markers in the low-risk population has to be established. For most of the patients with sonographically detected markers for congenital anomalies, the definitive invasive tests are not being done at most of the centers except for few highly specialized centers. We need to correlate our findings with biochemical markers and autopsy study. This is not done routinely because of socioeconomic constraints. The rate of abortion in the fetus with aneuploidy is relatively high and hence many of the fetuses are lost even before their first ultrasound screening. Being an operator-dependent modality, quality control and training in USG with registration and if possible grading of sonographers should be done in a specialized field like this to ensure high detection rate and standardization of practice. Last but not the least, expertise must match the technical advancement and the unavailability of high end USG units at most of the community and government healthcare set-up is an important limiting factor. CONCLUSION The application of prenatal ultrasound in aneuploidy risk assessment is a very valuable tool and based on the exponential increase in the number of research studies being published and technical advancement in the ultrasound units, the value is only going to increase further. Ultrasound allows patients to obtain more personalized risk assessment and has allowed many women a reasonable alternative to invasive test. The best estimate of risk seems to be achieved through the combined use of ultrasound, maternal serum screening and maternal age. As with many screening tests, it occasionally misses the diagnosis; this shortcoming must be well-understood. In the right hands and with appropriate counseling, ultrasound is an excellent tool. Despite its limitations, it provides information that is unmatched to other screening tests, especially when we are dealing with important issues like aneuploidy. After all, it is a teamwork that matters. Since an entire generation of geneticians, obstetricians and perinatologists are working hard to provide a eugenic society, the onus is on the ultrasonologist and imaging specialists to assistant their clinical colleague in this task. REFERENCES Wald NJ, Cuckle HS, Densem JW, Nanchahal K, Royston P, Chard T, et al. Maternal serum screening for Down s syndrome in early pregnancy. BMJ 1988;297(6653): Palomaki GE, Knight GJ, McCarthy J, Haddow JE, Eckfeldt JH. Maternal serum screening for fetal Down syndrome in the United States: a 1992 survey. Am J Obstet Gynecol 1993;169(6): Haddow JE, Palomaki GE, Knight GJ, Williams J, Pulkkinen A, Canick JA, et al. Prenatal screening for Down s syndrome with use of maternal serum markers. N Engl J Med 1992;327(9): Randomised trial to assess safety and fetal outcome of early and midtrimester amniocentesis. The Canadian Early and Mid-trimester Amniocentesis Trial (CEMAT) Group. Lancet 1998;351(9098): Wald NJ, Densem JW, George L, Muttukrishna S, Knight PG. Prenatal screening for Down s syndrome using inhibin-a as a serum marker. Prenat Diagn 1996;16(2): Wald NJ, Kennard A, Hackshaw A, McGuire A. Antenatal screening for Down s syndrome. J Med Screen 1997;4(4): Brigatti KW, Malone FD. First-trimester screening for aneuploidy. Obstet Gynecol Clin North Am 2004;31(1):v, Nicolaides KH, Azar G, Byrne D, Mansur C, Marks K. Fetal nuchal translucency: ultrasound screening for chromosomal defects in first trimester of pregnancy. BMJ 1992;304(6831): Philippe Jeanty. Lecture series on congenital anomalies. Greco P, Loverro G, Vimercati A, Marzullo A, Caruso G, Selvaggi L. Pathological significance of first-trimester fetal nuchal oedema. Prenat Diagn 1996;16(6): von Kaisenberg CS, Nicolaides KH, Brand-Saberi B. Lymphatic vessel hypoplasia in fetuses with Turner syndrome. Hum Reprod 1999;14(3): Indian Journal of Clinical Practice, Vol. 22, No. 11, April 2012

6 Moscoso G. Fetal nuchal translucency: a need to understand the physiological basis. Ultrasound Obstet Gynecol 1995;5(1):6-8. Hyett J, Moscoso G, Nicolaides K. Increased nuchal translucency in trisomy 21 fetuses: relationship to narrowing of the aortic isthmus. Hum Reprod 1995;10(11): Pandya PP, Johnson SP, Malligianis P, Nicolaides KH. First trimester fetal nuchal translucency and screening for chromosomal abnormalities. In: Ultrasound and Early pregnancy. Jurkovic, Jauniaux (Eds.), Parthenon Publishing, London and New York 1996:p Assessment of risks. In: Ultrasound Markers for Fetal Chromosomal Defects. Sniijders RJ, Nicolaides KH (Eds.), 1996:p Pandya PP, Goldberg H, Walton B, Riddle A, Shelley S, Snijders RJ, et al. The implementation of first-trimester scanning at weeks gestation and the measurement of fetal nuchal translucency thickness in two maternity units. Ultrasound Obstet Gynecol 1995;5(1):20-5. Hafner E, Schuchter K, Liebhart E, Philipp K. Results of routine fetal nuchal translucency measurement at weeks in 4233 unselected pregnant women. Prenat Diagn 1998;18(1): Nicolaides KH, Brizot ML, Snijders RJ. Fetal nuchal translucency: ultrasound screening for fetal trisomy in the first trimester of pregnancy. Br J Obstet Gynaecol 1994;101(9): Bewley S, Roberts LJ, Mackinson AM, Rodeck CH. First trimester fetal nuchal translucency: problems with screening the general population. 2. Br J Obstet Gynaecol 1995;102(5): Cha ban FK, Van Splunder P, Los FJ, Wladimiroff JW. Fetal outcome in nuchal translucency with emphasis on normal fetal karyotype. Prenat Diagn 1996;16(6): Reynders CS, Pauker SP, Benacerraf BR. First trimester isolated fetal nuchal lucency: significance and outcome. J Ultrasound Med 1997;16(2): 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 weeks of gestation. Fetal Medicine Foundation First Trimester Screening Group. Lancet 1998;352(9125): Scott F, Boogert A, Sinosich M, Anderson J. Establishment and application of a normal range for nuchal translucency across the first trimester. Prenat Diagn 1996;16(7): Nancy Walsh. Nuchal translucency helps identify aneuploidy in high-risk patients. (Brief Article): An article from: Family Practice News, Pandya PP, Brizot ML, Kuhn P, Snijders RJ, Nicolaides KH. First-trimester fetal nuchal translucency thickness and risk for trisomies. Obstet Gynecol 1994;84(3): Benn PA. Advances in prenatal screening for Down syndrome: II first trimester testing, integrated testing, and future directions. Clin Chim Acta 2002;324(1-2): Pandya PP, Snijders RJ, Psara N, Hilbert L, Nicolaides KH. The prevalence of non-viable pregnancy at weeks of gestation. Ultrasound Obstet Gynecol 1996;7(3): Cicero S, Curcio P, Papageorghiou A, Sonek J, Nicolaides K. Absence of nasal bone in fetuses with trisomy 21 at weeks of gestation: an observational study. Lancet 2001;358(9294): Wald NJ, Hackshaw AK. Combining ultrasound and biochemistry in first-trimester screening for Down s syndrome. Prenat Diagn 1997;17(9): Otaño L, Aiello H, Igarzábal L, Matayoshi T, Gadow EC. Association between first trimester absence of fetal nasal bone on ultrasound and Down syndrome. Prenat Diagn 2002;22(10): Viora E, Masturzo B, Errante G, Sciarrone A, Bastonero S, Campogrande M. Ultrasound evaluation of fetal nasal bone at 11 to 14 weeks in a consecutive series of 1906 fetuses. Prenat Diagn 2003;23(10): Cicero S, Rembouskos G, Vandecruys H, Hogg M, Nicolaides KH. Likelihood ratio for trisomy 21 in fetuses with absent nasal bone at the week scan. Ultrasound Obstet Gynecol 2004;23(3): Favre R, Cherif Y, Kohler M, Kohler A, Hunsinger MC, Bouffet N, et al. The role of fetal nuchal translucency and ductus venosus Doppler at weeks of gestation in the detection of major congenital heart defects. Ultrasound Obstet Gynecol 2003;21(3): Murta CG, Moron AF, Avila MA, Weiner CP. Application of ductus venosus Doppler velocimetry for the detection of fetal aneuploidy in the first trimester of pregnancy. Fetal Diagn Ther 2002;17(5): Bilardo CM, Müller MA, Zikulnig L, Schipper M, Hecher K. Ductus venosus studies in fetuses at high risk for chromosomal or heart abnormalities: relationship with nuchal translucency measurement and fetal outcome. Ultrasound Obstet Gynecol 2001;17(4): Matias A, Montenegro N. Ductus venosus blood flow in chromosomally abnormal fetuses at 11 to 14 weeks of gestation. Semin Perinatol 2001;25(1):32-7. Zoppi MA, Putzolu M, Ibba RM, Floris M, Monni G. First-trimester ductus venosus velocimetry in relation to nuchal translucency thickness and fetal karyotype. Fetal Diagn Ther 2002;17(1):52-7. Mavrides E, Sairam S, Hollis B, Thilaganathan B. Screening for aneuploidy in the first trimester by assessment of blood flow in the ductus venosus. BJOG 2002;109(9): Borrell A, Martinez JM, Serés A, Borobio V, Cararach V, Fortuny A. Ductus venosus assessment at the time of nuchal translucency measurement in the detection of fetal aneuploidy. Prenat Diagn 2003;23(11): Levi CS, Lyons EA, Lindsay DJ. Ultrasound in the first trimester of pregnancy. Radiol Clin North Am 1990;28(1): Levi CS, Lyons EA, Lindsay DJ. Early diagnosis of nonviable pregnancy with endovaginal US. Radiology 1988;167(2): Lyons EA, Levi CS. Ultrasound in the first trimester of pregnancy. Radiol Clin North Am 1982;20(2): Indian Journal of Clinical Practice, Vol. 22, No. 11, April

7