系列遗传学检测法评估真性胎儿染色体嵌合体的价值

目的探讨染色体核型分析、染色体微阵列分析(chromosomal microarray analysis, CMA)及荧光原位杂交(fluorescencein situ hybridization, FISH)技术在真性胎儿染色体嵌合体产前诊断中的应用价值。方法 2018年4月至2021年8月, 有明确产前诊断指征并在中山大学附属第一医院行羊膜腔穿刺术或/和脐静脉穿刺术产前诊断的单胎妊娠孕妇共4 071例, 其中产前诊断为胎儿真性染色体嵌合体的40例孕妇纳入回顾性分析。分析其染色体核型分析、CMA及FISH结果及嵌合染色体分布、嵌合比例及妊娠结局。采用χ2检验进行统计学分析。结果 (1)真性胎儿染色体嵌合体的检出率为0.98%(40/4 071)。(2)性染色体嵌合体占42.5%(17/40), 其他染色体嵌合体包括21、22、18、16、7、12、15、17和20号染色体及染色体平衡易位嵌合。(3)真性胎儿染色体嵌合体在羊水染色体核型分析中的检出率为77.4%(24/31);在羊水CMA中的检出率为76.7%(23/30);在脐血染色体核型分析中的检出率为10/19;在脐血CMA中...     

超数Y染色体49,XYYYY核型胎儿的产前诊断并文献复习

目的探讨对核型为49,XYYYY非嵌合体的超数Y染色体胎儿的产前诊断遗传咨询。方法患者孕18周,因血清学产前筛查为唐氏综合征高风险于2020年6月1日在云南省第一人民医院医学遗传科进行遗传咨询,知情同意后抽取胎儿羊水,应用G显带染色体核型分析技术和低覆盖度高通量测序技术对未经培养的羊水样本做拷贝数变异测序,进行遗传学分析和产前诊断。结果胎儿羊水细胞染色体核型为49,XYYYY,未经培养的胎儿羊水样本拷贝数变异测序结果为:seq[GRCh37](1-22)×2,(X)×1,(Y)×4,两种遗传学分析技术的检测结果均提示此胎儿为非嵌合体的4个Y染色体,即超数Y染色体。查询文献,全球共有6例49,XYYYY核型非嵌合体报道,其中5例为出生后患者,均有智力及语言发育障碍等较严重的临床表型。经产前诊断遗传咨询后,本例孕妇和家属自愿选择终止妊娠。结论超数Y染色体的49,XYYYY在胎儿期无超声可见的异常,但非嵌合体患者出生后可能出现智力和语言发育迟缓、特殊面容及骨骼发育异常等表型,可导致智力损害的严重出生缺陷,需在产前诊断遗传咨询中加以重视。     

联合细菌人工染色体微珠技术和染色体核型分析在二孩高龄孕妇产前诊断中的应用研究

目的:探讨联合细菌人工染色体微珠技术(BoBs)和染色体核型分析在二孩高龄孕妇产前诊断中的应用。方法:选择2016年8月至2018年8月在本院遗传咨询门诊、胎儿医学门诊及孕产妇保健门诊就诊的1291例二孩高龄孕妇为研究对象,对羊水细胞的染色体进行核型分析和BoBs分析,对胎儿染色体异常及常见微缺失综合征进行诊断。结果:1291例二孩高龄孕妇羊水样本中,染色体核型分析和产前BoBs均检出染色体异常47例。产前BoBs技术共检测出61例染色体异常,包括30例21-三体,14例18-三体,3例13-三体,14例染色体微缺失/微重复,总体检出率为4.73%,漏检13例,检测失败7例;染色体核型分析检测出60例染色体异常,比BoBs额外检出10例胎儿染色体结构异常,2例低比例的嵌合型染色体以及1例标记染色体,染色体核型分析染色体异常检出率为4.65%;经两者联合检出异常81例,联合检出率为6.27%。染色体核型分析和产前BoBs共同检出的47例二孩高龄孕妇选择了终止妊娠;BoBs漏检的13例胎儿染色体结构异常均为平衡易位或倒位,经遗传咨询后均选择了继续妊娠。BoBs检测失败的7例,经遗传咨询后继续妊娠。染色体核型分析漏检的14例经遗传咨询后均选择了终止妊娠。1291例二孩高龄孕妇均获得随访,其中经染色体核型分析和BoBs检测显示正常的1210例二孩高龄孕妇,胎儿分娩后均为正常健康胎儿;BoBs漏检的13例胎儿染色体结构异常在随访中均未见异常。结论:"核型分析+BoBs"产前诊断模式可以应用于二孩高龄孕妇的产前诊断,值得临床推广和应用。     

324例性染色体非整倍体产前诊断分析及遗传咨询

目的:初步探讨胎儿性染色体非整倍体异常核型的检出率及临床特征,分析不同产前诊断指征与性染色体非整倍体异常核型的关系,研究性染色体嵌合现象及妊娠结局,为临床医生遗传咨询提供指导.方法:选取2014年1月至2019年12月在空军军医大学西京医院妇产科接受产前诊断的孕妇14959例,进行羊水染色体核型分析和荧光原位杂交技术(...     

胎儿淋巴水囊瘤与染色体异常的相关性研究及妊娠结局分析

目的:探讨胎儿颈部淋巴水囊瘤(CCH)与染色体异常的相关性,以及早孕期CCH胎儿的妊娠结局。方法:选取2012年1月1日至2016年6月1日在广州医科大学附属第三医院胎儿医学科门诊行早孕期NT筛查发现CCH的胎儿160例。孕妇均行产前诊断,对染色体及基因芯片结果未见异常的胎儿,建议孕妇于妊娠20～24周接受详细的胎儿III级排畸超声筛查。新生儿随访时间为出生后2～25个月。结果:160例行遗传学诊断的早孕期CCH样本中,染色体核型分析44例,行高分辨CMA分析116例。染色体异常82例,其中最常见的染色体异常为Turner's综合征(31例,33.7%),其次为18三体(24例,26.1%)、21三体(17例,18.5%),13三体7例,染色体平衡易位2例,染色体嵌合1例。基因芯片异常10例,其中6例致病性拷贝数异常。染色体及芯片未见异常的68例中,除10例死胎外,发现超声结构异常18例。染色体、芯片及超声均未见异常且选择继续妊娠的病例中,活产率为98%。截止随访日,活产胎儿暂未发现明显异常。胎儿心脏异常尤其是室间隔缺损在非染色体异常病例中的发生率较高。26例双胎中,约57.7%发现明显染色体异常,染色体检查、芯片及超声检查均未见异常的病例中,选择继续妊娠的4例双胎均活产健康。结论:对于早孕期CCH的预后,需排除常见的染色体异常及致病性拷贝数变异,同时还需结合胎儿排畸超声情况综合判断。如均未见明显异常,且后期未发生胎儿水肿的孤立性淋巴水囊瘤的胎儿出生后预后良好。     

妊娠中期胎儿染色体病的产前诊断

目的 通过对妊娠中期高危孕妇羊水细胞染色体的核型分析，了解胎儿染色体核型异常发生情况。方法抽取1983年3月至2003年8月河南省人民医院342例符合产前诊断指征的妊娠中期孕妇羊水细胞进行培养，制备中期细胞染色体，用C、G、Q、R带等多种显带技术，进行染色体分析。结果在342例孕妇羊水中发现23例染色体异常，占6．7％，其中数目异常5例(21．8％)，嵌合体3例(13．0％)，结构异常15例(65．2％)。结论产前诊断胎儿染色体病最终仍需羊水染色体核型分析来确诊。     

3015例遗传咨询者外周血染色体核型嵌合体分析

目的 探究遗传咨询者外周血染色体核型嵌合类型、嵌合比例及其与临床表型的关系。方法回顾性分析3015例来自山东省妇幼保健院遗传咨询者外周血染色体核型,分析不同嵌合类型、嵌合比例个体的身高、体重、智力及生殖能力。结果 3015例外周血样本中染色体核型嵌合体27例,嵌合比例0.9%。致病性嵌合体21例,多态性嵌合体6例。致病性嵌合型中,X染色体嵌合体19例,多于常染色体嵌合体数量。X重复型嵌合体身高大于X丢失型嵌合体(P=0.032)。X嵌合体嵌合比例3%～5%,显著低于其他类型嵌合比例(P=0.000;χ²=23.5)。结论 纳入研究的样本中X染色体较常染色体嵌合比例更高;X染色体低比例嵌合体在所有嵌合体中占比大,且可能会影响个体身高发育。     

羊水染色体多态性与妊娠结局的关系分析

目的:探讨羊水染色体多态性与妊娠结局的关系,为产前诊断染色体多态性的临床处置提供理论依据。方法:对3960例高危孕妇行羊膜腔穿刺术,抽取羊水经培养后制备染色体核型并进行分析,诊断为染色体多态性的胎儿,其父母接受外周血染色体检查,并对1岁龄婴儿期生长发育情况进行跟踪随访。结果:3960例羊水染色体核型共检出多态性核型116例,其中114例多态性核型来自于父母其中一方,仅1例9qh-和1例22ps+为新生变异。随访发现110例胎儿孕中晚期及出生后1岁以内,未见明显生长发育异常;1例inv(9)于孕29+2周不明原因胎死宫内;1例新生9qh-胎儿,孕晚期B超发现胎儿头围、腹围较实际孕周小2周,出生8个月后随访身高、体重及运动协调能力发育较同龄儿稍低;另一例新生变异22ps+,孕期及出生后随访未见明显异常;4例失访,妊娠结局未知。结论:具有明确遗传来源的染色体多态性变异可参照其父辈的身体智力发育情况予以判断其妊娠结局,新发生的胎儿多态性变异对其妊娠结局及今后的生长发育情况可能造成一定的负面影响,但其影响的具体机制及对应关系还有待进一步研究。     

产前诊断指征在胎儿染色体异常诊断中的价值探讨

目的:探讨产前诊断指征在胎儿染色体异常诊断中的价值及其对妊娠结局的指导意义.方法:对439例有产前诊断指征的孕妇,在超声引导下经腹羊膜腔穿刺抽取羊水检查染色体核型,比较不同产前诊断指征的胎儿染色体异常检出率,分析各组染色体异常类型与妊娠结局的关系.结果:①胎儿染色体异常检出15例,总的异常检出率3.42%.夫妇平衡易位组胎儿染色体异常检出率最高为66.67%,与高龄组、唐氏高危组、不良孕产史(夫妇染色体检查正常)组比较,差异有统计学意义(P<0.05);而高龄组、唐氏高危组、不良孕产史组和超声检查异常组的胎儿染色体异常检出率分别为5.22%、2.28%、1.54%、16.67%、,组间两两比较差异均无统计学意义(P>0.05).②15例染色体异常中.高龄组占40.00%,唐氏高危组占33.33%.染色体数目异常6例,5例行孕中期引产;结构异常7例,1例行孕中期引产,1例流产;嵌合体2例均行孕中期引产;余6例足月分娩.结论:对具有产前诊断指征的孕妇进行羊水细胞培养及染色体核型分析,不仅能及时发现胎儿染色体异常,为孕妇是否继续妊娠提供科学依据,而且有利于降低出生缺陷发生率.     

20号染色体假性嵌合体的产前诊断

<正>在羊水培养过程中,20号染色体嵌合体并不多见。目前进一步的检测方法多是进行脐带血染色体分析,其结果可能为嵌合体,也可能为正常纯合体。鉴于羊水中培养的脱落细胞多为成纤维细胞,脐带血培养检测的是血液中的淋巴细胞,还有其它组织来源的细胞,不同组织细胞的染色体结果可能并不一致。本文现对经脐带血穿刺检查显示为正常纯合核型,而在羊水培养中提示为嵌合体的假性嵌合体的病例报告如下。     

Pseudomosaicism, true mosaicism, and maternal cell contamination in amniotic fluid processed with in situ culture and robotic harvesting.

This study was designed to test the usefulness of the common definitions for maternal cell contamination, true mosaicism, and pseudomosaicism for amniotic fluid specimens processed by in situ culture and robotic harvesting. We prospectively studied 4309 consecutive amniotic fluid specimens processed with these methods and found that 0.84 per cent had maternal cell contamination, 0.28 per cent had true mosaicism, and 5.4 per cent had pseudomosaicism. Although the frequencies of maternal cell contamination and true mosaicism were comparable to those in similar published studies, the frequency of pseudomosaicism was more than twice as high as that in previous reports. This finding is most likely not due to the method, but rather to a more accurate estimate of the actual frequency of pseudomosaicism in amniotic fluid cultures than reported heretofore. Follow-up clinical information was available on 72 per cent of the cases. In three cases of true mosaicism involving structural anomalies, the results of cytogenetic follow-up studies on the neonates were normal. None of the pseudomosaic cases involving trisomy 8, 13, 18, or 21; triple X; or monosomy X were associated with newborns who had birth defects.     

The dilemma of chromosomal mosaicism in chorionic villus sampling--'direct' versus long-term cultures

Chromosomal mosaicism is one of several unanswered dilemmas in first-trimester prenatal diagnosis. We report the course of a pregnancy in which a normal karyotype was detected on direct CVS preparation and fetal blood, 100 per cent trisomy 21 in one long-term CVS culture, and low-rate trisomy 21 mosaicism in a second long-term CVS culture and amniocentesis. The phenotypically normal infant had a 6 per cent mosaicism of trisomy 21. It appears that a persistent low-rate mosaicism in different tissues may be indicative of the true status of the fetus.     

Proposed guidelines for diagnosis of chromosome mosaicism in amniocytes based on data derived from chromosome mosaicism and pseudomosaicism studies.

Currently, accepted protocol which has been developed at the Prenatal Diagnosis Laboratory of New York City (PDL) requires that when a chromosome abnormality is found in one or more cells in one flask, another 20-40 cells must be examined from one or two additional flasks. Chromosome mosaicism is diagnosed only when an identical abnormality is detected in cells from two or more flasks. In a recent PDL series of 12,000 cases studied according to this protocol, we diagnosed 801 cases (6.68 per cent) of single-cell pseudomosaicism (SCPM), 126 cases (1.05 per cent) of multiple-cell pseudomosaicism (MCPM), and 24 cases (0.2 per cent) of true mosaicism. Pseudomosaicism (PM) involving a structural abnormality was a frequent finding (2/3 of SCPM and 3/5 of MCPM), with an unbalanced structural abnormality in 55 per cent of SCPM and 24 per cent of MCPM. We also reviewed all true mosaic cases (a total of 50) diagnosed in the first 22,000 PDL cases. Of these 50 cases, 23 were sex chromosome mosaics and 27 had autosomal mosaicism; 48 cases had numerical abnormalities and two had structural abnormalities. Twenty-five cases of mosaicism were diagnosed in the first 20 cells from two flasks, i.e., without additional work-up, whereas the other 25 cases required extensive work-up to establish a diagnosis (12 needed additional cell counts from the initial two culture flasks; 13 required harvesting a third flask for cell analysis). Our data plus review of other available data led us to conclude that rigorous efforts to diagnose true mosaicism have little impact in many instances, and therefore are not cost-effective. On the basis of all available data, a work-up for potential mosaicism involving a sex chromosome aneuploidy or structural abnormality should have less priority than a work-up for a common viable autosomal trisomy. We recommend revised guidelines for dealing with (1) a numerical versus a structural abnormality and (2) an autosomal versus a sex chromosome numerical aneuploidy. Emphasis should be placed on autosomes known to be associated with phenotypic abnormalities. These new guidelines, which cover both flask and in situ methods, should result in more effective prenatal cytogenetic diagnosis and reduced patient anxiety.     

Trisomy 7 mosaicism at amniocentesis: interphase FISH, QF-PCR, and aCGH analyses on uncultured amniocytes for rapid distinguishing of true mosaicism from pseudomosaicism

ObjectiveTo present prenatal diagnosis of true trisomy 7 mosaicism.Materials, Methods and ResultsA 36-year-old woman underwent amniocentesis at 18 weeks of gestation. Amniocentesis revealed a karyotype of 47,XY,+7[20]/46,XY[9]. The parental karyotypes were normal. Repeated amniocentesis was performed at 20 weeks of gestation. Array comparative genomic hybridization (aCGH) analysis on uncultured amniocytes manifested a genomic gain in chromosome 7. Quantitative fluorescent polymerase chain reaction (QF-PCR) analysis on uncultured amniocytes showed a biparental diallelic pattern with a dosage increase in the maternal allele. Interphase fluorescence in situ hybridization (FISH) on uncultured amniocytes revealed three 7q-specific signals in 13 of 50 (26%) of the cells. The cultured amniocytes had a karyotype of 47,XY,+7[12]/46,XY[14]. The ultrasound findings were unremarkable. The pregnancy was subsequently terminated, and a fetus was delivered with facial dysmorphisms. Postnatal tissue samplings revealed the mosaic trisomy 7 level of 37.5% (15/40), 30% (12/40), 42.5% (17/40), 82.5% (33/40), 52.5% (21/40), and 27.5% (11/40) in skin, liver, lungs, placenta, membrane, and cord, respectively. The cord blood had a karyotype of 46,XY. PEG1/MEST methylation-sensitive high-resolution melting PCR assay of cord blood showed no uniparental disomy for chromosome 7.ConclusionInterphase FISH, QF-PCR, and aCGH analyses on uncultured amniocytes are useful for rapid distinguishing of true mosaicism from pseudomosaicism for trisomy 7 at amniocentesis. Cord blood sampling for confirmation of fetal trisomy 7 mosaicism is not practical.     

Management of prenatally detected trisomy 8 mosaicism.

We report on ten pregnancies with trisomy 8 mosaicism. Nine cases were prenatally detected in chorionic villi (n=6), amniotic fluid (AF) cells (n=2) or fetal blood (FB) lymphocytes (n=1). Follow-up laboratory investigations showed confined placental mosaicism (CPM) or pseudomosaicism in eight cases. In one case with ultrasound abnormalities, trisomy 8 mosaicism was detected in FB cells although cultured AF cells showed normal cells only. Another case of mosaic trisomy 8 was prenatally missed; cytogenetic analysis of short-term cultured villi revealed a normal male karyotype, while postnatally, trisomy 8 mosaicism was detected in peripheral blood lymphocytes and skin fibroblasts of the affected child. These findings indicate the difficulties in the prenatal diagnosis of trisomy 8 mosaicism. When found in chorionic villi, it mostly represented CPM, while in a case of true fetal trisomy 8 mosaicism, the cytotrophoblast cells showed a normal karyotype. So, the cytotrophoblast compartment of chorionic villi is a poor indicator of the presence or absence of fetal trisomy 8 mosaicism. Follow-up investigations including amniocentesis and especially fetal blood sampling are required to come to a definite prenatal diagnosis of trisomy 8 mosaicism.     

Mosaicism or pseudomosaicism: the problem of hypermodal cells in amniotic fluid cell culture

A series of 2029 consecutive amniotic fluid specimens studied for prenatal genetic diagnosis were reviewed and reassessed so as to evaluate the frequency and clinical significance of hypermodal cells in amniotic fluid cell cultures. Hypermodal cells were defined as those with more than 46 chromosomes, and were characterized by an additional structurally normal or structurally abnormal chromosome. Of 2029 specimens, 47 (2.31 per cent) contained a total of 167 hypermodal cells. True fetal mosaicism was detected in three cases (0.14 per cent). All had hypermodal cells in more than one culture flask or colony which contained the same aberrant chromosome complement. In all but one case the babies were normal when only one cell was hypermodal, or when several cells were hypermodal but present in only one colony or one culture vessel. One case had an extra No. 20 chromosome in one cell. Although the child had multiple anomalies, they were not characteristic of trisomy 20, and subsequent chromosomal study on the baby postnatally revealed a 46,XX karyotype. The in situ coverslip technique is recommended as the preferred method for prenatal diagnosis, and it is useful as an aid in differentiating true mosaicism from pseudomosaicism.     

Follow-up investigations in uncultured amniotic fluid cells after uncertain cytogenetic results

Fluorescence in situ hybridization (FISH) on uncultured amniotic fluid (AF) cells is a widespread technique for the rapid prenatal detection of specific chromosome aberrations. During a 6-year period (1993-1998) we used FISH for quick follow-up investigations in uncultured AF cells after finding an uncertain chromosome aberration in a first chorionic villus (CV) or AF sample in 79 cases. These FISH results were compared with conventional cytogenetic results of the AF cell cultures in all cases. We found discrepant FISH and cytogenetic results in four instances. In general, FISH on uncultured AF cells proved to be a reliable technique for the rapid differentiation between confined placental mosaicism and true fetal mosaicism, and between pseudomosaicism and true mosaicism, respectively. Uncultured cells may sometimes even better reflect chromosomal mosaicism than cultured cells, since they are not subject to culture induced selection mechanisms. However, we found evidence that exceptional cases of tissue confined mosaicism may go undetected in uncultured cells.     

Prenatal diagnosis of pseudomosaicism for trisomy 20 at amniocentesis with a negative non-invasive prenatal testing (NIPT) result in a pregnancy with a favorable outcome

ObjectiveWe present prenatal diagnosis of pseudomosaicism for trisomy 20 at amniocentesis with a negative non-invasive prenatal testing (NIPT) result in a pregnancy with a favorable outcome.Case reportA 33-year-old, primigravid woman underwent amniocentesis at 17 weeks of gestation, which revealed a karyotype of 47,XX,+20[8]/46,XX[31]. Simultaneous array comparative genomic hybridization (aCGH) analysis on the DNA extracted from uncultured amniocytes revealed the result of arr (1–22,X) × 2, consistent with no genomic imbalance. She was referred to the hospital for repeat amniocentesis at 23 weeks of gestation. At repeat amniocentesis, cultured amniocytes had a karyotype of 47,XX,+20[2]/46,XX[33]. The parental karyotypes were normal. Simultaneous aCGH analysis on the DNA extracted from uncultured amniocytes using SurePrint G3 Unrestricted CGH ISCA v2, 8 × 60 K (Agilent Technologies, Santa Clara, CA, USA) revealed no genomic imbalance, or arr (1–22,X) × 2, Y × 0. Interphase fluorescence in situ hybridization (FISH) analysis using the bacterial artificial chromosome (BAC) probes of RP11-266K16 [20q13.33; fluorescein isothiocyanate (FITC), spectrum green] and RP11-348I14 (20q11.1-q11.21; Texas Red, spectrum red) detected trisomy 20 signals in 4/104 uncultured amniocytes (3.8%), compared with 0/100 in the normal control. Polymorphic DNA marker analysis using the DNA extracted from uncultured amniocytes and parental bloods excluded uniparental disomy 20. NIPT analysis on maternal blood revealed a negative result without gene dosage increase in chromosome 20. The pregnancy was carried to term, and a healthy 2830-g female baby was delivered with no phenotypic abnormality. Both cord blood and placenta had a karyotype of 46,XX.ConclusionNIPT is useful for rapid differential diagnosis of pseudomosaicism from true mosaicism in case of mosaic trisomy 20 at amniocentesis.     

Chromosomal mosaicism in chorionic villus sampling

The observation of multiple, chromosomally distinct cell lines in chorionic villus samples is not an unusual finding and occurs in 1 per 100 samples. This frequency is ten times greater than the level of mosaicism observed in newborn surveys and, thus, must reflect phenomenon other than true fetal mosaicism. Indeed, only 23% of mosaicism detected at CVS is confirmed in the fetus (2.3 per 1,000 CVS), which is much closer to the newborn rate (1 per 1,000). This indicates that most mosaicism encountered in CVS is unrelated to the fetal karyotype and as such is an inaccurate prediction of the fetal genotype, the purpose of prenatal diagnosis. Most of the mosaicism detected in CVS is due to confined placental mosaicism. Either as a result of error-prone cell division generating an excess of abnormal cells in extraembryonic tissues or reduced selection against aneuploid cells in these tissues allowing their persistence, chorionic villi and placenta appear to show much higher levels of mosaicism than seen in fetuses. This explains the more frequent finding of multiple cell lines in CVS than in amniocentesis or liveborn individuals. The discrepancy between levels of mosaicism present in chorionic villi and fetal tissues means that most instances of mosaicism detected in CVS are not associated with a fetal abnormality and should be evaluated by further prenatal testing, i.e., amniocentesis or fetal blood sampling. Because of the frequency of chromosomal mosaicism in CVS and its attendant need for further testing, a discussion of mosaicism should be included in counseling prior to CVS. The higher frequency of discrepant results in direct CVS preparation emphasizes the prudence of delaying decision making until the results of the CVS culture have been obtained. Although the observation of mosaicism clearly complicates genetic counseling and decision making, it does not appear to be associated with an adverse fetal outcome. Whereas most of the mosaicism observed in CVS is the result of confined placental mosaicism, other types of discrepancies also occur. Maternal cell contamination occurs in about 1% of cases, but is easily evaluated by examining the direct preparation and analyzing chromosome polymorphism. The incidence of pseudomosaicism in CVS cultures is unclear but probably low. Interestingly, CVS analysis has suggested that twinning may be a more common phenomenon at conception than reported at birth and that some discrepancies may reflect the nonviability of twins with abnormal karyotypes. Chorionic villi sampling remains a viable alternative to amniocentesis for early prenatal diagnosis. An understanding of the origins of mosaicism in CVS is necessary for     

The significance of trisomy 7 mosaicism in chorionic villus cultures

Two cases of mosaic trisomy 7 confined to the cultured cells and not found in direct preparation were detected from 200 consecutive first-trimester chorionic villus samples (CVS) analysed. The mosaicism was similar in the two cases, but the pregnancy outcome was different. In both cases, the direct metaphases from the CVS were 46,XY. Culture metaphases were mos46,XY/47,XY, +7; the trisomy 7 was seen in 34 per cent of cells from case 1 and 53 per cent from case 2. A sonogram at 15 1/2 weeks revealed fetal death in utero in case 1, and the patient declined amniocentesis. The fetal tissue failed to grow in culture, but the placental cultured cells were 47,XY, +7 in 28 (100 per cent) cells analysed. In the second case, all the amniotic fluid cells were 46,XY and the pregnancy resulted in a normal male with a 46,XY karyotype in the cord blood and foreskin fibroblast cultures. The term placenta was mosaic with 13/163 (8 per cent) trisomy 7 cells. Extensive cytogenetic studies on the placenta for the first time confirmed trisomy 7 mosaicism confined to the villus cultures.