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The 10% rate of preterm birth rate worldwide has not been proved amenable to reduction. Avoiding multiple embryo transfer in assisted reproductive technologies (ART) using in vitro fertilization is one unassailable method. Preimplantation genetic testing (PGT) to select only a single euploid embryo for transfer is one unequivocal way, maintaining 50%-60% pregnancy rates while avoiding twins. Contemporary methodology entails trophectoderm biopsy of a 5-6-day blastocyst, and cryopreservation of biopsied embryos while awaiting analysis by next generation sequencing. Embryo biopsy is safe, analytic validity for chromosomal analysis high, and global access to PGT high., (© 2020 International Federation of Gynecology and Obstetrics.)

Disorders of genetic etiology exist in 2%-3% of live-born infants. Identifying couples with increased susceptibility for offspring with anomalies or genetic disorders is increasingly effective as a result of molecular advances. Preimplantation genetic testing (PGT) with the use of trophectoderm biopsy, 24-chromosome testing, and molecular testing have allowed wider applicability for avoiding a clinical pregnancy termination. Cell-free DNA in maternal blood is another targeted option, although invasive prenatal genetic diagnosis provides the greatest amount of genetic information. DNA-based methods to detect subtle chromosomal abnormalities are much more sensitive than traditional karyotypes and do not require cultured cells. Aneuploidy and structural chromosomal abnormalities can be readily detected with the use of small amounts of DNA, if necessary amplified, as in PGT. Novel approaches exist for detecting perturbations in single-gene disorders. Not only has the molecular basis for many monogenic disorders been elucidated, but modest costs for DNA sequencing has made testing feasible. As the number of testable genetic disorders has increased, principles underlying screening have advanced. Genetic screening for disorders of high incidence in certain ethnic groups was initiated decades ago; however, limitations exist, and reduction in live-born incidence is not infrequently small. Expanded carrier screening is now offered in panethnic fashion, extending surveillance to couples of mixed ethnicities and involving many more genetic conditions. Targeted gene panels (e.g., adult-onset cancer genes) further increase the number of genetic disorders amenable to screening, often leading to PGT., (Copyright © 2019 American Society for Reproductive Medicine. Published by Elsevier Inc. All rights reserved.)

Preimplantation genetic diagnosis (PGD) can be considered the earliest form of prenatal testing. It was first used in humans over 26 years ago. At its inception, PGD could only be performed for a limited number of genetic disorders. Technological advances in molecular biology and cytogenomics have been utilized in the field of PGD to greatly expand the spectrum of genetic disorders that can now be detected in early human embryos.

Couples at risk for autosomal recessive congenital adrenal hyperplasia often request anticipatory guidance and genetic counseling. Initially, hormones in amniotic fluid were measured to distinguish affected female fetuses from unaffected fetuses. With the molecular era, more-targeted approaches became possible. Prenatal genetic diagnosis via amniocentesis or chorionic villus sampling was used to determine the need for continuing fetal therapy (dexamethasone), allowing cessation if the fetus was unaffected. Newer methods now allow diagnosis earlier in gestation, further shortening the treatment time for unaffected female fetuses who will not develop genital ambiguity. Preimplantation genetic testing permits transfer only of an unaffected female or male fetus. Analysis of maternal cell-free DNA based on quantitative differences in the amount of allele parental DNA permits affected pregnancies to be differentiated from unaffected pregnancies., (Copyright © 2018. Published by Elsevier Inc.)

Biomarkers can be employed for screening for fetal genetic disorders, identifying individuals at sufficiently high risk for a confirmatory invasive procedure. In this article we discuss prenatal genetic aneuploidy screening. Maternal serum analytes and ultrasound have long been routinely offered, providing detection rates of 80% to 93% for trisomy 21; however, an invasive procedure (false-positive) must be performed in 5% of pregnancies screened. Recovering fetal cells from maternal blood initially proved inconsistent, but new methods recovering trophoblasts offer promise for detecting disorders using a single fetal cell, analogous to preimplantation genetic diagnosis. Current emphasis is focused predominantly on cell-free fetal DNA, which accounts for 5% to 10% of total cell-free DNA in maternal blood. Analysis of maternal blood results in detection rates of over 99% for fetal trisomy 21, and also very high rates for trisomy 18 and sex chromosomal abnormalities. Such detection rates are substantively higher than with maternal serum analytes, and are accompanied by a much lower (<1%) false-positive rate., (Copyright © 2013. Published by Elsevier Inc.)

Background: Chromosomal microarray analysis has emerged as a primary diagnostic tool for the evaluation of developmental delay and structural malformations in children. We aimed to evaluate the accuracy, efficacy, and incremental yield of chromosomal microarray analysis as compared with karyotyping for routine prenatal diagnosis., Methods: Samples from women undergoing prenatal diagnosis at 29 centers were sent to a central karyotyping laboratory. Each sample was split in two; standard karyotyping was performed on one portion and the other was sent to one of four laboratories for chromosomal microarray., Results: We enrolled a total of 4406 women. Indications for prenatal diagnosis were advanced maternal age (46.6%), abnormal result on Down's syndrome screening (18.8%), structural anomalies on ultrasonography (25.2%), and other indications (9.4%). In 4340 (98.8%) of the fetal samples, microarray analysis was successful; 87.9% of samples could be used without tissue culture. Microarray analysis of the 4282 nonmosaic samples identified all the aneuploidies and unbalanced rearrangements identified on karyotyping but did not identify balanced translocations and fetal triploidy. In samples with a normal karyotype, microarray analysis revealed clinically relevant deletions or duplications in 6.0% with a structural anomaly and in 1.7% of those whose indications were advanced maternal age or positive screening results., Conclusions: In the context of prenatal diagnostic testing, chromosomal microarray analysis identified additional, clinically significant cytogenetic information as compared with karyotyping and was equally efficacious in identifying aneuploidies and unbalanced rearrangements but did not identify balanced translocations and triploidies. (Funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and others; ClinicalTrials.gov number, NCT01279733.).

Purpose of Review: For over a decade, methods of first-trimester, noninvasive prenatal genetic diagnosis have been actively pursued by many investigators. Isolation of fetal trophoblast from endocervical specimens remains an attractive approach, given the greater numbers of fetal cells than in maternal blood and the better potential for fetal-cell identification based on markers specific for a single cell type (trophoblasts)., Recent Findings: Current studies demonstrate feasibility in identification and molecular analysis of fetal trophoblast cells for prenatal genetic testing. Sampling methods involving lavage, cytobrush, or aspiration of cervical specimens, however, have limitations in the recovery of trophoblasts., Summary: Clinical applications await further systematic studies to determine safety and accuracy in recovery.

The three respondents provide additional support for preimplantation genetic diagnosis (PGD) having the pivotal place it now has in prenatal genetic diagnosis: chromosomal abnormalities (e.g., unbalanced translocations), Mendelian disorders, and HLA typing for transfer of compatible, genetically normal, embryos. Transferring euploid embryos has decreased the clinical abortion rate and increased the implantation rate in assisted reproductive technologies (ART), but it has not necessarily improved the live-birth rate. Safer embryo biopsy, more extensive diagnostic efforts (i.e., microarray analysis), and more refined patient selection may be required before shifting from preselection of embryos based solely on morphological parameters to transfer of only aneuploidy-free embryos.

Genetic counseling can best be performed if a systematic approach is taken. The geneticist or genetic counselor has the difficult task of conveying complex information to patients in an understandable form. At the same time, this must be done in a nondirective manner, affording the patient the right to make his or her own decisions. Obtaining accurate family and medical history information is crucial to the genetic counseling process. Given the nature of genetic information, multiple sessions may be necessary to ensure that the patient understands the risks involved in his or her particular situation.

The importance of the recent recommendations that address cystic fibrosis carrier screening cannot be overemphasized. For the first time, a systematic approach to offering or making screening available to all pregnant women in the hopes of providing refined risk estimates for a genetic disease has been established. Caucasian of European or Ashkenazi-Jewish descent should be offered screening. Within the proposed guidelines are ethnic-specific carrier frequencies (1/29) used to establish who should be offered testing and to whom testing should be made available. Recent recommendations have made clear that in a pan-ethnic population a frequency of 1/1,000 is required for inclusion into the cystic fibrosis mutation panel. A general framework for screening during pregnancy has been established (either concurrent or sequential). It will be interesting to watch as the fruits of the human genome project are inspected and applied to everyday clinical practice. No doubt the cost of screening will be reduced through advances in technology. The combined efforts of NIH, ACOG, and ACMG have provided the first set of comprehensive standards for screening of recessive diseases. How time changes these guidelines deserves following.

Fetal cells exist in maternal blood and can be utilized for prenatal genetic diagnosis. These cells are present during the first and second trimesters, with frequency increasing as gestation advances. Enrichment of erythroblasts by various density gradient techniques and either magnetic activated or flow sorting techniques can be followed by FISH with chromosome-specific DNA probes. This approach has allowed detection of trisomy 21, trisomy 18, Klinefelter syndrome 47,XXY and 47,XYY. Polymerase chain reaction (PCR) analysis of maternal blood has enabled the detection of fetal sex, certain Mendelian disorders (e.g. beta-globin mutations), HLA polymorphisms, and fetal Rhesus (D) blood type. The fetal cell types receiving the most attention has been nucleated erythrocyte (erythroblast) and the trophoblast. Lymphocytes and granulocytes are also present in maternal blood; however, lymphocytes are considered the cell type most likely to persist after pregnancy. A large scale collaborative is now underway in the U.S. that will allow determine whether sensitivity and specificity of this technique provide a noninvasive alternative to conventional methods of prenatal cytogenetic diagnosis.

During the last two decades a number of methods of prenatal diagnosis have become available and have been used either in laboratory research or in routine genetic counselling. Despite the effectiveness of invasive sampling procedures in diagnosing genetic disorders, their use involves some risk. The advantage of noninvasive methods is that they provide an opportunity to make a genetic diagnosis without risk, and therefore are applicable for use in mass screening programmes. This article reviews three different approaches to noninvasive prenatal genetic diagnosis and offers conclusions and recommendations for their use. Maternal serum screening is a well-understood technique that should be universally offered to pregnant women, regardless of their risk status. Invasive tests can be used, as indicated, once serum testing results have been obtained. Although ultrasonography cannot be recommended for routine use, it can provide a useful adjunct to serum screening and deserves further investigation. Elaboration of fetal cells from maternal blood is a promising technique but can only be considered investigational on the basis of current research, and should not serve as the sole basis of clinical decision-making.

Advances in molecular genetics have made possible advances in two new areas of prenatal diagnosis. Preimplantation genetic diagnosis is being explored through polar body biopsy, biopsy of the single cell from the eight-cell embryo, and trophectoderm biopsy of the blastocyst. There are attractive theoretical advantages to the last approach. However, assessment of pregnancies to be continued has been attempted only in the former two, and has succeeded only in the eight-cell embryo. A major problem with polar body biopsy is recombination for loci near the centromere, which may make analysis of the first polar body alone uninformative. DNA amplification of a single cell by polymerase chain reaction also has inherent pitfalls. Fetal cells in maternal circulation have ostensibly been shown to exist: lymphocytes, trophoblasts, and erythroblasts. The basis for this claim is polymerase chain reaction amplification showing fetal DNA sequences in maternal blood. Diagnosis of fetal chromosomal abnormalities requires techniques other than polymerase chain reaction, namely fluorescence in situ hybridization. Fetal trisomy has thus been detected by flow-sorting maternal blood for fetal erythroblasts followed by subjecting sorted cells to fluorescence in situ hybridization for chromosome-specific probes.

Screening for genetic abnormalities is an integral part of obstetrics. Prior to initiating screening, however, several prerequisites must be met: (i) capacity to alter clinical management, (ii) cost effectiveness, (iii) reliable means (usually assays) of assessment, and (iv) capacity to handle problems. In all pregnancies one should determine in systematic fashion whether family history places a pregnant woman at increased risk over the background risk of 2-3% congenital anomalies. All women over age 35 years at delivery should be offered prenatal cytogenetic testing, and women of all ages should be offered maternal serum alpha-fetoprotein screening for neural tube defects. Screening ostensibly normal populations is appropriate in certain ethnic groups to determine heterozygosity for selected disorders: Blacks for sickle-cell anaemia, Mediterranean people for beta-thalassaemia, Southeast Asians and Filipinos for alpha-thalassaemia, Ashkenazi Jews and perhaps French-Canadians for Tay-Sachs disease. Cystic fibrosis screening (delta F508 mutations) is not currently recommended for the general populations, but should be offered to relatives of an individual having delta F508 cystic fibrosis. Irrespective of the extent of screening programmes for Mendelian traits, the mutant allele will remain in the general population because by far the greatest genetic load lies in clinically normal heterozygotes, affected contributing far less to the load despite the obvious clinical effect.

We present our initial experience in developing a chorionic villus sampling program at Northwestern University. In phase 1, we performed chorionic villus sampling in 58 patients prior to elective first-trimester abortion, assessing the reliability and reproducibility of obtaining adequate villus samples and performing cytogenetic analysis by means of both the direct and culture methods. Specimens were categorized according to quality: class I, multiple identifiable villi (n = 20); class II, few villi or villi mixed with decidua (n = 15); class III, no villi (n = 23). There was a positive trend between operator experience, amount of villi obtained, and quality of cytogenetic preparations. In March, 1984, we received Institutional Review Board approval to perform chorionic villus sampling in continuing pregnancies (phase 2). Among the first 20 cases we found two abnormalities (47,XY, + 13; 45,X). The remaining 18 pregnancies were continuing. Recommendations are made for developing a chorionic villus sampling program.

One of important problems in the health care application of technological developments in genomics is the availability of prospective identification of those at need for these technologies. The development and implementation of prospective carrier screening programs in the last few years is expected to increase the uptake of PGT-M. But most of couples at genetic risk are unaware of their genetic profiles to request PGT, which has been improved with the upgrade of traditional ethnicity-based screening to the expanded carrier screening (ECS). Recent advances in genomic technologies allow for multiplexed platforms with ability to screen for thousands of mutations and variants at a reasonable cost. The present ECS panels differ greatly in scope and include over thousand conditions and variants of variable penetrance, which are commercially available, offering the possibility of a more comprehensive and efficient risk evaluation. However, although the potential ECS to increase the number of couples presenting for PGT-M is obvious, no data are available to evaluate their actual impact. So this report evaluates the impact of ECS on the uptake of PGT-M, based on our ongoing PGT-M experience, which represents the world's largest series in one center. A total of 643 (54.6%) PGT-M cases were performed through ECS referral during the years 2016-2018, as part of our overall PGT-M practice of 1188 cases, including referrals through traditional approach during the same period of time. To evaluate the impact of ECS on the PGT-M uptake, we analyzed the dynamics of increase of at-risk couples presenting for PGT-M through ECS overall, as well as separately for each genetic condition tested, compared to the baseline referrals through the traditional approach. The analysis showed an increase of ECS based referrals to PGT-M from 35.1% (133 of 322 cases) in 2016 to 53.8% (210 of 392 total) in 2017, and 63.3% (300 of 474) in 2018. Among the most frequently referred conditions were cystic fibrosis (CFTR) (131 of 182, 72%), the uptake of which increased from 50% in 2016 to 82.7% in 2018; Deafness (GLB2) (65 of 81, 80.2%) with the uptake growing from 31.2% in 2015 to 86.4% in 2018; Fragile X (FMR1) (102 of 128, 80%), the uptake growing from 73% in 2016 to 84% in 2018; Thalassemia and sickle cell disease (HBB) (58 of 99, 58.6%), increasing from 45.8% in 2016 to 60.9% in 2018. So, the overall number of prospective PGT-M cases for the last three years more than doubled after ECS-based referral, with a similar dynamics for each condition tested (up to 80% increase of referrals through ECS). This may become the major source for performing PGT-M in the near future, allowing to offer PGT-M prospectively before the birth of an affected child. The data show significant increase of the PGT-M uptake following ECS, demonstrating the utility for offering PGT-M prospectively to the couples at risk. [ABSTRACT FROM AUTHOR]