The NT value measured by 11–13 + 6W ultrasound is an important indicator for evaluating fetal chromosomal abnormalities in the early pregnancy . Studies have shown that , when the NT value is 3.4mm, the fetal chromosome abnormality rate is only 3.7% when NT ≥ 3.5mm, the fetal chromosome abnormality rate is as high as 21.1%. In this study, among 240 samples with NT ≥ 2.5mm, the fetal chromosome abnormality rate was 20.75% (50/241). Among them, trisomy 21 syndrome accounted for 50% (25/ 50), 18 trisomy syndrome accounted for 10% (5/ 50), Turner syndrome accounted for 10% (5/ 50), and other chromosome abnormalities accounted for 30% (15/ 50). The results suggest that NT thickening is closely related to chromosome abnormalities, especially trisomy 21 syndrome. Souka et al showed that when NT ≥ 3.5mm, the risk of chromosome abnormality increased exponentially with the thickening of NT . Maya et al reported that the fetal chromosome abnormality rates in NT ≤ 2.9 mm, 3.0-3.4mm and ≥ 3.5mm groups were 1.7%, 6.5% and 13.8% respectively . In this study, the fetal chromosome abnormality rates of the six groups with NT values < 2.5mm, 2.5-2.9mm, 3.0-3.4mm, 3.5-4.4mm, 4.5-5.4mm and ≥ 5.5mm were 0.8%, 11.63%, 17.81%, 20%, 47.62% and 63.64% respectively. The above results showed that the thicker the NT was, the higher the abnormal rate of fetal chromosome was, which is consistent with the previous literature. At present, most studies [16–18] set the cutoff value for prenatal diagnosis of NT thickening as ≥ 3.5mm. Maya and Lena [15, 19] suggest that fetuses with NT ≥ 3.0mm should be evaluated for prenatal diagnosis. However, the chromosome abnormality rate of fetuses with NT value of 2.5-2.9mm was 11.63% in this study, which was significantly higher than 0.8% of NT value < 2.5mm, and the difference was statistically significant. Therefore, more experimental data are needed to support the criteria for determining the critical value of NT thickening for prenatal diagnosis. Our results support the evaluation of invasive prenatal diagnosis in pregnant women with NT ≥ 2.5mm.
The indications of interventional prenatal diagnosis include not only increased NT value, but also the elderly, adverse fertility history, abnormal ultrasound, high risk of serological screening, and chromosomal abnormalities in both husband and wife, etc . In the study of XueShuya , the rate of chromosomal abnormalities in fetuses with simple NT thickening was 22.6 %, while the rate of chromosomal abnormalities combined with other ultrasound abnormalities such as lymphocystoma, nasal bone deficiency or fetal edema was 60.9 %. In this study, according to the different indications of prenatal diagnosis, 241 cases of NT thickening were divided into NT thickening alone and NT thickening combined with other abnormalities (fetal nasal bone dysplasia, cervical lymphocystoma, subcutaneous tissue thickening, edema, choroid plexus cyst, single umbilical artery, ventricular hyperintense spot, elderly ,adverse fertility history,etc.). The abnormal rates of fetal chromosomes were 13.17% and 35.14%, respectively. The rate of fetal chromosomal abnormalities is higher when NT thickening combined with other abnormalities, so when NT thickening combined with other ultrasonic abnormalities in early pregnancy, it is more necessary to exclude chromosomal abnormalities in time for prenatal diagnosis.
At present, CMA detection is more and more used in prenatal diagnosis, and many studies have confirmed the correlation between NT thickening and chromosome microdeletion or microrepetitive variation [22, 23]. Grande et al conducted a meta-analysis of 17 studies and concluded that about 4% more pathogenic chromosome microdeletions or microrepetitive variations could be detected by CMA technology in fetuses with normal karyotype analysis . Egloff et al reported that in fetuses with NT thickening in early pregnancy, CMA could detect about 2.7 % of pathogenic chromosome variations that cannot be detected by conventional karyotype analysis . In this study, the karyotype analysis and CMA of chromosome abnormality detection rate in 366 samples were 13.39 % ( 49 / 366 ) and 13.93 % ( 51 / 366 ), respectively, with no significant difference. However, CMA can bring us more data information, which can correlate genotype with phenotype,and complement each other with karyotype analysis to facilitate clinical consultation. In this study, karyotype analysis showed that 5 samples had complex chromosomal rearrangements and 1 sample had additional marker chromosomes, CMA results not only supplemented the karyotype results, suggesting that five samples had chromosome microdeletions and duplications at the same time, clarifying the source of one marker chromosome (Fig. 2), Fig. 2.Ⅰ was the CMA result and Fig. 2.Ⅱ was the karyotype result. CMA results but also supplemented the genotype was associated with phenotype, giving us more information. In view of the complex chromosome structure rearrangement of five samples, we verified the peripheral blood chromosomes of three samples of parents and found that two cases were new mutations and one case was caused by maternal inheritance (Fig. 1), Fig. 1.Ⅰand Ⅱ were the CMA result of fetus, Fig. 1.Ⅲ was the karyotype result of fetus, Fig. 1.Ⅳ was the karyotype result of mother.We also classified all CNVs and followed up the pregnancy: 10 cases of pathogenic CNVs (1), 7 cases were detected by karyotype analysis and CMA, and pregnant women chose induced labor after informed choice. (2) 3 cases were detected by CMA but not detected by karyotype analysis. 1 case with a 5.37Mb deletion of 9q22.32q31.1, the related phenotypes included epileptic encephalopathy, language development disorder, macrosomia, learning disability and nephroblastoma. Ultrasound also revealed multiple fetal abnormalities: cervical fold thickening, bilateral cleft lip and palate, posterior fossa cystic structure, and bilateral renal separation. The pregnant woman decided to induce labor after informed choice. 1 case with a 1.4 Mb deletion of 17p12, whose pathogenicity was related to hereditary pressure susceptible peripheral neuropathy. The CMA of parents confirmed that it was caused by the father, and no abnormality was found in ultrasound. After careful consideration, the pregnant woman chose to continue pregnancy, and has delivered a healthy baby.1 case with a 2.49Mb duplication of 22q11.21,the repetition in this region involves 22q11.2 microrepetition syndrome, but it is an incomplete explicit syndrome, with a penetrance rate of about 21.9%. The related abnormal phenotypes include developmental retardation and mental retardation, hypodystonia, autistic behavior, congenital heart abnormalities, and so on. Considering the normal ultrasound examination and the existence of explicit insufficiency, the pregnant woman carefully chose to continue the pregnancy, and now she has given birth to a normal fetus, about 4 months old, and the cardiac ultrasound examination is normal. 2 cases of probable pathogenic CNVs (1) 1 case with a 394Kb deletion of 1q21.1, this region is the susceptible site of thrombocytopenia-radial deficiency syndrome, the main phenotypes are thrombocytopenia, bilateral radius loss, cardiac and urinary system abnormalities and so on. The syndrome follows the autosomal recessive genetic model. In addition to the single copy deletion in the region, which is a possible pathogenic CNV, gene mutations can also be found in most patients. However, ultrasound examination is normal, the pregnant women carefully choose to continue pregnancy, and now she has given birth to a normal fetus, about 2 months old, heart, urinary and other ultrasound examination is normal. (2) 1 case with a 5.09Mb deletion of 8q22.3q23.1, involving 17 OMIM genes such as ZFPM2.The mutation of ZFPM2 gene is related to cardiac abnormalities of autosomal dominant inheritance such as tetralogy of Fallot and double outlet of right ventricle. and heterozygous mutations of ZFPM2 gene can also be found in patients with partial reversal syndrome. The deletion of ZFPM2 gene can be derived from parents with normal phenotype, and the deletion is a possible pathogenic CNV.Fetal ultrasound and other tests are normal, pregnant women carefully choose to continue pregnancy, has now given birth to a normal fetus, more than 20 days, cardiac ultrasound examination is normal. 15 cases of VOUS:1 case lost follow-up information, 14 cases of ultrasound and other examinations were normal, It is suggested that the source of VOUS should be confirmed by fetal parents, but only 2 cases were verified by their parents, and the results were all caused by heredity. Pregnant women chose to continue pregnancy carefully and have given birth to healthy fetuses. Most fetuses with VOUS have better pregnancy outcome, but we still need to combine ultrasound and other comprehensive assessment, and carry out long-term and detailed clinical follow-up of born babies to accumulate more clinical information.
Domenico et al showed that  when the NT value was 3.5-4.4mm, about 70% of the fetuses had no obvious abnormality until delivery; when the NT value was 4.5-5.4mm, only 50% had no obvious abnormality; when the NT value was 5.5-6.4mm, only 30% had no obvious abnormality; when the NT value was ≥ 6.5mm, only 15% had no obvious abnormality. Souka et al pointed out that  NT thickening is not only related to chromosome abnormalities, but also with adverse perinatal outcomes caused by various fetal malformations, dysplasia, rupture and genetic syndrome. However, when chromosome problems were excluded and the fetus survived to metaphase without any abnormality in ultrasound, the risk of poor perinatal outcome and postnatal growth retardation did not increase statistically. In this paper, 18 fetuses with ≥ 5.0mm were followed up. (1) 9 cases of chromosome abnormalities were detected by karyotype analysis and CMA. Pregnant women chose induced labor after informed choice. (2) in 5 cases, only early ultrasound showed thickening of NT, normal ultrasound such as anaphase aberration, normal karyotype analysis and CMA. Pregnant women chose to continue pregnancy carefully and all gave birth to normal fetuses. (3) 1 case with a 524Kb accication of 3q29, the meaning of repetition in this area was unknown, the parents verified that the repetition was inherited from the father with normal phenotype, and the ultrasound examination was normal, the pregnant woman carefully chose to continue the pregnancy, and now she has given birth to a normal fetus, 18 months old. (4) 1 case of early ultrasound showed that the cervical fold thickened by NT 6.1mm, system ultrasound showed that the thickness of cervical fold is about 9mm and the other was normal, chromosome karyotype analysis and CMA were normal, and the pregnant woman chose to continue the pregnancy cautiously, and now she has given birth to a normal fetus, more than 2 years old. (5) 1 case showed NT8.0mm and cervical and dorsal lymphoid cystic tumor in the early stage, normal ultrasound in the middle and later stage, normal chromosome karyotype analysis and CMA. The pregnant woman carefully chose to continue the pregnancy, and now she has given birth to a normal fetus for more than 7 months. (6) 1 case of early ultrasound revealed NT 5.0mm and local skin edema of the chest and abdominal wall of the fetus. In the second trimester, the width of bilateral ventricles of the fetus was about 9mm, chromosome karyotype analysis and CMA detection were normal, and the width of the left ventricle of the fetus in the late pregnancy was about 9mm. Noonan and other syndromes were ruled out by gene detection in the whole exon group. The pregnant woman chose carefully to continue the pregnancy, and now she has given birth to a normal fetus for more than 3 months. Among the 18 fetuses, 9 fetuses had normal chromosomes and no obvious abnormalities in ultrasound, and all of them had a good pregnancy outcome. Therefore, even if the NT is thickened, the fetus usually has a better pregnancy outcome when there are no obvious abnormalities in chromosome, whole exon gene detection and middle and late ultrasound, which should be treated with caution.