Preimplantation genetic diagnosis and chromosome analysis of blastomeres using comparative genomic hybridization

Numerical chromosome errors are known to be common in early human embryos and probably make a significant contribution to early pregnancy loss and implantation failure in IVF patients. Over recent years fluorescent in situ hybridization (FISH) has been used to document embryonic aneuploidies. Many IVF laboratories perform preimplantation genetic diagnosis (PGD) with FISH to select embryos that are free from some aneuploidies in an attempt to improve implantation, pregnancy and live birth rates in particular categories of IVF patients. The usefulness of FISH is limited because only a few chromosomes can be detected simultaneously in a single biopsied cell. Complete karyotyping at the single cell level can now be achieved by comparative genomic hybridization (CGH). CGH enables not only enumeration of all chromosomes but gives a more complete picture of the entire length of each chromosome and has demonstrated that chromosomal breakages and partial aneuploidies exist in embryos. CGH has provided invaluable information about the extent of mosaicism and aneuploidy of all chromosomes in early human conceptuses. CGH has been applied to clinical PGD and has resulted in the birth of healthy babies from embryos whose full karyotype was determined in the preimplantation phase.     

Preimplantation genetic diagnosis for an insertional translocation carrier

BACKGROUND: While preimplantation genetic diagnosis (PGD) is well established for carriers of reciprocal terminal translocations, reports on PGD for insertional translocation carriers are lacking. Here, we report on the PGD of an insertional translocation carrier with karyotype 46,XX,ins(14;2)(q21;q31q35). Due to the possibility of crossovers within the inserted region, rather than a single probe, four probes are required for proper embryo selection. METHODS: Probes were generated for PGD using fluorescence in situ hybridization and two PGD cycles. RESULTS: Analysis of 10 embryos revealed four embryos to be normal diploid. Two embryos were consistent with 3:1 segregation of the theoretical quadrivalent and one was consistent with 2:2 or 1:1 segregation. Furthermore, one embryo was mosaic abnormal and one remained without diagnosis. CONCLUSIONS: With increased acceptance of PGD, it is likely that more carriers of complex translocations will enter PGD programmes. The present results suggest that a careful genetic work-up of complex translocations is essential for proper embryo selection. While theoretical modelling may predict that quadrivalents will form during the meiosis of insertional translocations, experimental proof for the occurrence of quadrivalents is still lacking and more research on the meiotic process of both female and male insertional translocation carriers is warranted.     

Robertsonian translocations--reproductive risks and indications for preimplantation genetic diagnosis.

BACKGROUND: Robertsonian translocations carry reproductive risks that are dependent on the chromosomes involved and the sex of the carrier. We describe five couples that presented for preimplantation genetic diagnosis (PGD). METHODS: PGD was carried out using cleavage-stage (day 3) embryo biopsy, fluorescence in-situ hybridization (FISH) with locus-specific probes, and day 4 embryo transfer. RESULTS: Couple A (45,XX,der(14;21)(q10;q10)) had two previous pregnancies, one with translocation trisomy 21. A successful singleton pregnancy followed two cycles of PGD. Couple B (45,XX,der(13;14)(q10;q10)) had four miscarriages, two with translocation trisomy 14. One cycle of PGD resulted in triplets. Couple C (45,XX,der(13;14)(q10;q10)) had four years of infertility; two cycles were unsuccessful. Couple D (45,XY,der(13;14)(q10;q10)) presented with oligozoospermia. A singleton pregnancy followed two cycles of PGD. Couple E (45,XY,der(13;14)(q10;q10)) had a sperm count within the normal range and low levels of aneuploid spermatozoa. PGD was therefore not recommended. No evidence for a high incidence of embryos with chaotic or mosaic chromosome complements was found. CONCLUSIONS: For fertile couples, careful risk assessment and genetic counselling should precede consideration for PGD. Where translocation couples need assisted conception for subfertility, PGD is a valuable screen for imbalance, even when the risk of viable chromosome abnormality is low.     

SNP array-based copy number and genotype analyses for preimplantation genetic diagnosis of human unbalanced translocations

Preimplantation genetic diagnosis (PGD) for chromosomal rearrangements (CR) is mainly based on fluorescence in situ hybridisation (FISH). Application of this technique is limited by the number of available fluorochromes, the extensive preclinical work-up and technical and interpretative artefacts. We aimed to develop a universal, off-the-shelf protocol for PGD by combining single-nucleotide polymorphism (SNP) array-derived copy number (CN) determination and genotyping for detection of unbalanced translocations in cleavage-stage embryos. A total of 36 cleavage-stage embryos that were diagnosed as unbalanced by initial PGD FISH analysis were dissociated (n=146) and amplified by multiple displacement amplification (MDA). SNP CNs and genotypes were determined using SNP array. Epstein-Barr Virus-transformed cell lines with known CR were used for optimising the genomic smoothing (GS) length setting to increase signal to noise ratio. SNP CN analysis showed 23 embryos (64%) that were unbalanced in all blastomeres for the chromosomes involved in the translocation, 5 embryos (14%) that were normal or balanced in all blastomeres and 8 embryos (22%) that were mosaic. SNP genotyping, based on analysis of informative SNP loci with opposing homozygous parental genotypes, confirmed partial monosomies associated with inheritance of unbalanced translocation in surplus embryos. We have developed a universal MDA-SNP array technique for chromosome CN analysis in single blastomeres. SNP genotyping could confirm partial monosomies. This combination of techniques showed improved diagnostic specificity compared with FISH and may provide more reliable PGD analysis associated with higher embryo transfer rate.     

Analysis using fish of sperm and embryos from two carriers of rare rob(13;21) and rob(15;22) robertsonian translocation undergoing PGD

The majority of fluorescence in situ hybridization (FISH) studies on the meiotic segregation of Robertsonian translocations focus on the most common types, rob(13; 14) and rob(14; 21). Here we report the first study for carriers of rare Robertsonian translocations rob(13; 21) and rob(15; 22) combining analysis of meiotic segregation in sperm and blastomeres following pre-implantation genetic diagnosis (PGD). Dual-colour FISH was applied to nuclei from spermatozoa and blastomeres from PGD embryos using two subterminal contig probes for each translocation, and a second round with probes for chromosomes 16 and 18. Patient 1 had a rob(13; 21) and patient 2 had a rob(15; 22), and 86.3% and 87.5% of gametes respectively were consistent with meiotic segregation resulting in a normal or balanced chromosome complement. Analysis of embryos showed that for patient 1 and 2 respectively, 25% and 46% were balanced, and of the unbalanced embryos, 50% and 31% were mosaic or chaotic. Our patients with a rob(13; 21) and rob(15; 22) were found to have a similar meiotic segregation pattern to that for male carriers of the common Robertsonian translocations. The observed rate in unbalanced embryos being mosaic or chaotic may result in an increased risk of chromosomal abnormalities. Our results may help to improve the genetic counseling for carriers of rare Robertsonian translocations.     

Aneuploidy 12 in a Robertsonian (13;14) carrier: Case report

In translocation carriers, the presence of aneuploidy for the chromosomes unrelated to the rearrangement may lead to an additional risk of abnormal pregnancy or implantation failure. Consequently, it may be important to analyse not only the chromosomes involved in the rearrangement but also the rest of chromosomes. We combined spectral karyotyping (SKY) and comparative genomic hybridization (CGH) to karyotype one unfertilized oocyte and its first polar body (1PB) from a Robertsonian translocation carrier t(13;14) aged 29 years who was undergoing IVF and preimplantation genetic diagnosis (PGD) for translocations and aneuploidy screening. Two out of four embryos were aneuploid, as a result of an adjacent segregation. The unfertilized oocyte had a normal/ balanced constitution of the chromosomes involved in the reorganization. However, this 1PB-metaphase II doublet was aneuploid for chromosome 12, the oocyte being hyperhaploid (24, X, +12) and its 1PB hypohaploid (22, X, -12). The application of CGH for the study of Robertsonian translocations of maternal origin will be useful to study imbalances of the chromosomes involved in the rearrangement, as well as alterations in the copy number of any other chromosome. The combination of PGD for translocations with aneuploidy screening could help to reduce the replacement of chromosomally abnormal embryos.     

The development of cytogenetically normal, abnormal and mosaic embryos: a theoretical model

Assisted reproduction and preimplantation genetic diagnosis (PGD) involve various complicated techniques, each of them with its own problems. However, the greatest problem with PGD for chromosome abnormalities is not of a technical nature but is a biological phenomenon: chromosomal mosaicism in the cleavage stage embryo. Here, we present a hypothetical, quantitative model for the development of chromosomally normal, abnormal and mosaic embryos. The arising of mosaicism in 2-8-cell embryos was described by a binomial probability model on the occurrence of mitotic events inducing chromosomal changes in the blastomeres. This model converted the 'mean' rate of mosaicism found in cross-sectional studies (60%) into an equal rate of mosaic embryos at arrival at the 8-cell stage (59.8%). The disappearance of > 90% of the mosaic embryos or the mosaicism itself from surviving embryos during the morula stage was explained by mitotic arrest of most of the mitotically changed cells under increasing cell cycle control. In our model, 25.9 and 14.3% of the embryos at the 8-cell stage are normal and abnormal respectively. The remaining 59.8% of the embryo shows mosaicism: 34.6% of abnormal/normal cells and 25.2% of abnormal/abnormal cells. The high proportion of abnormal and mosaic embryos together explains the high rate of abnormal laboratory findings in PGD for chromosomal abnormalities and aneuploidy screening. The poor representation of a 1- or 2-cell biopsy for the 7- or 6-cell post-biopsy embryo in the case of mosaicism explains the high rate of false-negative and false-positive results.     

Successful PGD cycles for mosaic Robertsonian translocation carriers provide insights into the mechanism of formation of the derivative chromosomes

Preimplantation genetic diagnosis (PGD) has been carried out for two couples with different mosaic Robertsonian translocations. Two PGD cycles for a mosaic 13;13 homologous Robertsonian translocation carrier resulted in the birth of a healthy child in each cycle, illustrating the importance of scanning G‐banded preparations from homologous Robertsonian carriers for the presence of a normal cell line. One couple was referred for PGD because the male partner carried a mosaic 14;15 Robertsonian translocation with a normal cell line. A single PGD cycle resulted in the birth of a healthy child. Follow‐up studies and extended FISH analysis of the carrier's lymphocytes detected three cell lines, two carrying different 14;15 Robertsonian chromosomes and one normal cell line. The two 14;15 Robertsonian chromosomes had different breakpoints in the proximal short arm regions. We suggest that the presence of the D15Z1 polymorphism on the short arm of one chromosome 14 mediated the post‐zygotic formation of the two different Robertsonian chromosomes. © 2013 Wiley Periodicals, Inc.     

Single intragenic microsatellite preimplantation genetic diagnosis for cystic fibrosis provides positive allele identification of all CFTR genotypes for informative couples

This study is part of a strategy aimed at using fluorescent polymerase chain reaction (PCR) on informative genetic microsatellite markers as a diagnostic tool in preimplantation genetic diagnosis (PGD) of severe monogenic disease. Two couples, both of whom had previously had children who were compound heterozygote for severe cystic fibrosis mutations, were offered PGD using fluorescent PCR of the highly polymorphic cystic fibrosis transmembrane conductance regulator (CFTR) intragenic microsatellite marker IVS17bTA. Cleavage-stage embryo biopsy followed by PCR resulted in transfer of one unaffected carrier embryo for each couple. This approach eliminates the need for single cell multiplex PCR strategies to detect CF compound heterozygotes. It also provides a control of chromosome 7 ploidy in the blastomeres and a selection against allele dropout by positive detection of each CFTR copy of all genotypes in preimplantation embryos from genetically informative families.     

The combination of polar body and embryo biopsy does not affect embryo viability

BACKGROUND: The biopsy of both polar bodies and a blastomere from the same embryo was investigated as an approach aimed at increasing the quantity of DNA available for genetic analysis in preimplantation embryos. METHODS: In 113 cycles, preimplantation genetic diagnosis (PGD) was performed for aneuploidy: 19 cycles underwent polar body biopsy, 32 cycles had both polar body and blastomere biopsy done, and the remaining 62 cycles underwent blastomere biopsy. The chromosomal analysis was performed in a two-round fluorescence in situ hybridization (FISH) protocol with probes specific for the chromosomes X, Y, 13, 15, 16, 18, 21 and 22. RESULTS: The morphological evaluation of the analysed embryos demonstrated similar rates of development irrespective of the biopsy procedure. Accordingly, the implantation rate did not differ significantly in the three biopsy groups and was 15% after polar body biopsy, 26% after the combined biopsy procedures of polar bodies and blastomeres, and 25% after blastomere biopsy. CONCLUSIONS: The removal of a blastomere subsequent to polar body biopsy does not seem to have negative effects on embryo viability. This approach could be especially valuable for a combined diagnosis of aneuploidy and single-gene disorders in preimplantation embryos generated by couples at high reproductive risk.     

大Y携带者不良孕产史与胚胎非整倍体率增高的关系

目的探讨1例有不良孕产史的大Y携带者的胚胎异常情况。方法1对有2次自然流产史的夫妇,男方染色体核型为46,XY,Yqh ,常规超促排卵和卵母细胞胞浆内单精子注射,受精后第3天和第4天进行胚胎活检,获取分裂球,采用18,X,Y三色着丝粒探针进行荧光原位杂交分析(FISH),第5天移植正常胚胎。异常胚胎及废弃胚胎所有分裂球第6天再次FISH确定胚胎核型。结果患者获卵19个,对其中13个M2期卵母细胞进行ICSI,12个受精,分裂11胚。10个胚胎获得明确诊断,其中4个正常胚胎,6个异常胚胎,异常发生率达60%。5个为女胚,其中1个正常核型,4个异常胚胎中2个为无序分裂,2个为嵌合体;5个为男胚,3个正常,2个异常胚胎中1个为无序分裂,1个为嵌合体。对染色体正常的1个女胚进行宫腔内移植,未获得妊娠。结论该例大Y患者胚胎非整倍体发生率增高可能是导致其不良孕产史的原因。     

Possible interchromosomal effect in embryos generated by gametes from translocation carriers

BACKGROUND: The incidence of abnormal pregnancies in carriers of balanced translocations depends strictly on the chromosomes involved in the translocations. The aim of this study was to verify whether conventional aneuploidy screening could be advantageously combined with preimplantation genetic diagnosis (PGD) for translocations. METHODS: Twenty-eight carriers of Robertsonian and reciprocal translocations underwent 43 PGD cycles; specific probes were used to screen the translocation in 172 embryos generated by 35 cycles; most of these embryos were also screened for chromosomes 13, 16, 18, 21, 22 (n = 166), XY (n = 107), 1 (n = 17) and 15 (n = 88). For the remaining eight cycles (carriers of reciprocal translocations) only the chromosomes involved in common aneuploidy screening were investigated on the 40 embryos generated in vitro. RESULTS: In Robertsonian translocations, the proportion of embryos with abnormalities due to the translocation was 21%, common aneuploidies contributed 31% of total abnormalities, whereas the remaining 36% of embryos had abnormalities due to both types of chromosome. For reciprocal translocations, the chromosomes involved in the translocation were responsible for 65% of total abnormalities; only 6% of the embryos were abnormal for common aneuploidies and 16% carried abnormalities due to both the chromosomes involved in the translocation and those not related to the translocation. CONCLUSIONS: An interchromosomal effect seems to play a role in the case of Robertsonian translocations, where the relevant contribution of aneuploidy exposes the couple to an additional risk of abnormal pregnancy.     

PGD for a complex chromosomal rearrangement by array comparative genomic hybridization

Patients carrying a chromosomal rearrangement (CR) have an increased risk for chromosomally unbalanced conceptions. Preimplantation genetic diagnosis (PGD) may avoid the transfer of embryos carrying unbalanced rearrangements, therefore increasing the chance of pregnancy. Only 7-12 loci can be screened by fluorescence in situ hybridization whereas microarray technology can detect genome-wide imbalances at the single cell level. We performed PGD for a CR carrier with karyotype 46,XY,ins(3;2)(p23;q23q14.2),t(6;14)(p12.2;q13) using array comparative genomic hybridization. Selection of embryos for transfer was only based on copy number status of the chromosomes involved in both rearrangements. In two ICSI-PGD cycles, nine and seven embryos were analysed by array, leaving three and one embryo(s) suitable for transfer, respectively. The sensitivity and specificity of single cell arrays was 100 and 88.8%, respectively. In both cycles a single embryo was transferred, resulting in pregnancy following the second cycle. The embryo giving rise to the pregnancy was normal/balanced for the insertion and translocation but carried a trisomy 8 and nullisomy 9 in one of the two biopsied blastomeres. After 7 weeks of pregnancy the couple miscarried. Genetic analysis following hystero-embryoscopy showed a diploid (90%)/tetraploid (10%) mosaic chorion, while the gestational sac was empty. No chromosome 8 aneuploidy was detected in the chorion, while 8% of the cells carried a monosomy for chromosome 9. In summary, we demonstrate the feasibility and determine the accuracy of single cell array technology to test against transmission of the unbalanced meiotic products that can derive from CRs. Our findings also demonstrate that the genomic constitution of extra-embryonic tissue cannot necessarily be predicted from the copy number status of a single blastomere.     

Rapid visualization of metaphase chromosomes in single human blastomeres after fusion with in-vitro matured bovine eggs

The present study was aimed to facilitate karyotyping of human blastomeres using the metaphase-inducing factors present in unfertilized eggs. A rapid technique for karyotyping would have wide application in the field of preimplantation genetic diagnosis. When cryopreserved in-vitro matured bovine oocytes were fused with human blastomeres, the transferred human nuclei were forced into metaphase within a few hours. Eighty-seven human blastomeres from abnormal or arrested embryos were fused with bovine oocytes in a preclinical study. Fusion efficiency was 100%. In 21 of the hybrid cells, no trace of human chromatin was found. Of the remaining 66, 64 (97%) yielded chromosomes suitable for analysis. The method was used to karyotype embryos from two patients with maternal translocations. One embryo which was judged to be karyotypically normal was replaced in the first patient, resulting in one pregnancy with a normal fetus. None of the second patient's embryos was diagnosed as normal, and hence none was transferred. The results of the present study demonstrated that the ooplasmic factors which induce and maintain metaphase in bovine oocytes can force transferred human blastomere nuclei into premature metaphase, providing the basis for a rapid method of karyotyping blastomeres from preimplantation embryos and, by implication, cells from other sources.     

First successful double‐factor PGD for Lynch syndrome: monogenic analysis and comprehensive aneuploidy screening

Preimplantation genetic diagnosis (PGD) has been applied worldwide for a great variety of single‐gene disorders over the last 20 years. The aim of this work was to perform a double‐factor preimplantation genetic diagnosis (DF‐PGD) protocol in a family at risk for Lynch syndrome. The family underwent a DF‐PGD approach in which two blastomeres from each cleavage‐stage embryo were biopsied and used for monogenic and comprehensive cytogenetic analysis, respectively. Fourteen embryos were biopsied for the monogenic disease and after multiple displacement amplification (MDA), 12 embryos were diagnosed; 5 being non‐affected and 7 affected by the disease. Thirteen were biopsied to perform the aneuploidy screening by short‐comparative genomic hybridization (CGH). The improved DF‐PGD approach permitted the selection of not only healthy but also euploid embryos for transfer. This has been the first time a double analysis of embryos has been performed in a family affected by Lynch syndrome, resulting in the birth of two healthy children. The protocol described in this work offers a reliable alternative for single‐gene disorder assessment together with a comprehensive aneuploidy screening of the embryos that may increase the chances of pregnancy and birth of transferred embryos.     

Reduced survival after human embryo biopsy and subsequent cryopreservation.

Preimplantation genetic diagnosis (PGD) is performed in couples at risk of genetic disease, so as to avoid transfer of embryos which are affected by a monogenic disease or which carry chromosomal aberrations. As in all in-vitro fertilization (IVF) cycles, supernumerary non-affected good-quality embryos may be available after PGD. These embryos can be cryopreserved. So far, limited data on survival after cryopreservation of biopsied human embryos are available. In this study, human embryos of good morphological quality derived from abnormal fertilization were used to evaluate the influence of the embryo biopsy procedure on survival after cryopreservation. Embryos were allocated to three different groups: control (n = 20), drilling-only (n = 16), and biopsy (n = 29). After freezing and thawing, a significantly lower number of blastomeres was intact in the drilling-only group (46/118, i.e. 39.0%, P < 0.01) and in the embryo biopsy group (46/156, i.e. 29.5%, P < 0.0001) than in the control group (85/151, i.e. 56.3%). This difference was reflected in survival rates of embryos. Fifty-five per cent of the control embryos, 37.5% of the drilling-only group, and 33.3% of the biopsy group had at least 50% of their blastomeres intact. After further in-vitro culture, four blastocysts, three from the drilling-only group and one from the biopsy group, developed from the surviving embryos. From this study it can be concluded that current cryopreservation procedures are less successful when biopsied human embryos are cryopreserved, but that surviving embryos can develop to the blastocyst stage and thus may have the potential to develop to term.     

Preimplantation genetic diagnosis and sperm analysis by fluorescence in-situ hybridization for the most common reciprocal translocation t(11;22).

In this study we describe the pre-clinical development and clinical application of preimplantation genetic diagnosis (PGD) by fluorescence in-situ hybridization (FISH) for two non-related carriers (one male and one female) of the most common balanced reciprocal translocation: t(11;22)(q25;q12). For the couple with the female carrier, enumeration of the sex chromosomes in the embryos was also indicated (husband: 47,XXY karyotype). Four-colour FISH analysis was performed on six blastomeres from three embryos. No embryo transfer was possible because all the embryos were unbalanced. Three PGD cycles, with two-colour FISH, were carried out for the couple with the male translocation carrier. A total of 35 embryos were biopsied and diagnosed by FISH; nine out of the 35 embryos (25. 7%) were normal and seven of them were transferred (two embryos from the first and four from the third cycle), six out of 35 embryos (17%) were unbalanced, three out of 35 embryos (5.7%) were triploid or polyploid, 10 out of 35 embryos (28.6%) were mosaic and seven out of 35 embryos (20%) were chaotic. Diagnosis failed in 2.9% of the embryos. The spermatozoa of the male carrier were also analysed using three-colour FISH. Only 29.1% of the sperm cells seemed to be balanced or normal. By choosing probes lying on both sides of the breakpoints and by using a combination of sub-telomeric or locus-specific probes and centromeric probes, the use of three-colour FISH enabled detection of all the imbalances in sperm and/or cleavage-stage embryos in the patients. This may improve risk assessment and genetic counselling in the future for translocation carriers.     

Meiotic outcomes in reciprocal translocation carriers ascertained in 3-day human embryos

Chromosomes involved in reciprocal translocations form quadrivalents at meiosis. These quadrivalents segregate, with or without recombination, to give 32 different meiotic outcomes, only two of which are normal or balanced. This paper presents data collected from 25 cycles of preimplantation genetic diagnosis for 18 couples carrying 15 different reciprocal translocations. Embryos were tested using fluorescence in situ hybridisation with probes for the translocated and centric segments. Overall, 47.7% (71 out of 149) of embryos tested showed signal patterns consistent with alternate segregation, 24.8% adjacent-1 segregation, 10.1% adjacent-2 segregation, 15.4% 3 : 1 segregation and 2% 4 : 0 segregation. For most translocations, alternate segregation was apparently the most frequent mode. Alternate and adjacent-1 frequencies were similar in male and female carriers; however, 5.7% of embryos from female translocation carriers showed adjacent-2 segregation and 20.0% showed 3 : 1 segregation, whilst the corresponding figures for male carriers were 20.5 and 4.5%. Overall, 2.8% of embryos were mosaic and 2.3% of embryos showed chaotic constitutions for the chromosomes tested. The pregnancy success rate for these 25 cycles was 38.8% per embryo transfer and also 38.8% per couple.     

Massively parallel sequencing on human cleavage-stage embryos to detect chromosomal abnormality

Purpose

Next-generation sequencing technology like MPS has recently been introduced to perform comprehensive chromosome screening on human trophectoderm samples for preimplantation embryo assessment. However, the potential of MPS in chromosome analysis of single cell from blastomeres has not yet been investigated.

Comprehensive chromosomal analysis of human preimplantation embryos using whole genome amplification and single cell comparative genomic hybridization

Analysis of small numbers of chromosomes using interphase fluorescent in-situ hybridization (FISH) probes has revealed that 50% of human preimplantation embryos contain abnormal cells. Detection of high levels of mosaicism with so few probes has led some researchers to extrapolate that a full analysis of all 23 pairs of chromosomes would reveal that all human embryos contain a proportion of abnormal cells. However, existing cytogenetic protocols cannot achieve such an analysis due to technical limitations. We have developed a novel technique based on whole genome amplification and comparative genomic hybridization (CGH), which for the first time allows the copy number of every chromosome to be assessed in almost every cell of a cleavage-stage embryo. We have successfully analysed 64 cells (blastomeres) derived from 12 embryos and have detected unusual forms of aneuploidy, high levels of chromosomal mosaicism, non-mosaic aneuploidy and chromosome breakage. This is the first report of a comprehensive assessment of chromosome copy number in human embryos and indicates that, despite high levels of mosaicism, some embryos do have normal chromosome numbers in every cell. Such embryos may have a superior developmental potential, and their low frequency may explain correspondingly low success rates of natural and assisted conception in humans.