1 The Role of the Obstetrician in the Prevention and Treatment of Birth Defects.- 1. Introduction.- 2. The Abnormal Conceptus.- 3. Causes of Birth Defects.- 4. Screening for Congenital Anomalies.- 4.1. Intrauterine Infections.- 4.2. Neural Tube Defects.- 4.3. Ultrasonography.- 5. Diagnosis of the Abnormal Conceptus.- 5.1. X-Ray.- 5.2. Ultrasound.- 5.3. Amniocentesis.- 5.4. Fetoscopy.- 5.5. Chorionic Villous Sampling.- 6. Management of the Abnormal Conceptus.- 6.1. Abortion.- 6.2. Fetal Surgery.- 7. The Future.- References.- 2 The Nature and Causes of Spontaneous Abortions with Normal Karyotypes.- 1. Introduction.- 2. Mechanisms of Abortion.- 3. Incidence of Chromosomally Normal Abortuses.- 4. Clinically Implicated Causes of Spontaneous Abortion.- 4.1. Infection.- 4.2. Immunological Factors and Spontaneous Abortion.- 4.3. Morphological Abnormalities.- 4.4. Abnormalities of the Uterine Environment and of Uterine Function.- 4.5. Endocrinological Factors.- 4.6. Maternal Disease and Spontaneous Abortion.- 4.7. Environmental Factors and Spontaneous Abortion.- 5. Summary.- 6. Conclusions.- References.- 3 Temporal Trends in Twinning.- 1. Introduction.- 2. Time Trends in Twinning Rates.- 2.1. Western Europe.- 2.2. Eastern Europe.- 2.3. United States.- 2.4. Canada.- 2.5. Japan.- 2.6. Other Countries.- 2.7. Summary: The Decline in Twinning in Recent Years.- 2.8. Variations in Twinning Rates over the Longer Term.- 2.9. Is the Drop in Rates Ending?.- 3. The Etiology of Twinning. Some Recent Findings.- 4. The Physiological Mechanism of DZ Twinning.- 5. Hypotheses Related to the Decrease in Twinning Rates.- 5.1. Age and Parity Distribution of the Maternal Population.- 5.2. The Differential Fecundability Hypothesis.- 5.3. Effects of Oral Contraceptives.- 5.4. Hypothesis of an Increase in Spontaneous Abortion.- 5.5. Decreased Sperm Counts or Sperm Quality.- 5.6. Coital Rates.- 5.7. Genetic Factors.- 5.8. Urbanization and Stress.- 5.9. Nutritional and Socioeconomic Factors.- 6. Conclusion.- References.- 4 Cytogenetic and Clinical Significance of Fragile Sites on Human Chromosomes.- 1. Introduction.- 2. Fragile Sites on Autosomes.- 2.1. Folate-Sensitive Fragile Sites.- 2.2. BUdr-Requiring Fragile Site 10q25.- 2.3. Fragile Site 16q22.- 2.4. Fragile Site 17pl2.- 2.5. Individual Folate-Sensitive Fragile Sites.- 2.6. Other, Not Yet Fully Accepted, Folate-Sensitive Fragile Sites.- 3. Fragile Site on the Human X Chromosome (Xq27).- 3.1. Hemizygotes.- 3.2. Heterozygotes.- 3.3. Cytogenetic Diagnostic Criteria.- 3.4. Amniotic Fluid Cell Cultures.- 3.5. Formal Genetics and Genetic Counseling.- 3.6. Location of the Fragile Site Mutation on the X Chromosome.- 4. X-Linked Mental Retardation without Cytogenetic Manifestation.- 5. Concluding Remarks.- References.- 5 Informative Morphogenetic Variants: Minor Congenital Anomalies Revisited.- 1. Background.- 2. Terminology; Definition.- 3. Quantification of Informative Morphogenetic Variants (IMV): Formal and Comparative Aspects.- 3.1. Properties of the Examination.- 3.2. Quality of the Examiners and Recording of Their Observations.- 3.3. Characteristics of the Population Examined.- 4. The Biology of Informative Morphogenetic Variation.- 5. The Specific and Nonspecific Value of Informative Morphogenetic Variants, Singly and in Combination.- 5.1. Single Informative Morphogenetic Variants.- 5.2. Combinations of Informative Morphogenetic Variants As Predictors, and Clues to the Temporal Origin and Pathogenesis of Defective Intellectual and/or Behavioral Development.- 6. The Use of Specific Aggregates of Informative Morphogenetic Variants for Diagnostic Purposes.- 7. The Application of Numerical Taxonomy to Informative Morphogenetic Variants for the Purposes of Classification and Nosology.- 8. Informative Morphogenetic Variants As Indices of Teratogenic Environments.- 9. How Can the Information Value of Morphogenetic Variants Be Increased?.- 9.1. Measurement of Graded Anthropometric Characters to Identify Informative Morphogenetic Variants.- 9.2. Recognition of Factors Affecting the Discriminative Value of Ungraded Informative Morphogenetic Variants.- 10. The Positive Prognostic and Dysmorphogenetic Significance of Informative Morphogenetic Variants.- 11. Informative Morphogenetic Variants As Components of Predictive Scores for Nonspecific Detection of Congenital Disorders Not Apparent at Birth.- 12. The Value of Informative Morphogenetic Variants in Late Childhood- or Adult-Onset Disease.- 12.1. Thoracic-Outlet Syndrome.- 12.2. Mitral-Valve Prolapse (MVP) Syndromes.- 12.3. Accessory Spleens.- 13. Informative Morphogenetic Variants As Sentinel Phenotypes for Monitoring Environmental Teratogenic Hazards in Populations.- 14. Summary.- 14.1. What Do We Know about Morphogenetic Variants That Is Useful Now?.- 14.2. What Do We Need to Know or Do to Increase the Informative Value of Morphogenetic Variants?.- 15. Epilogue.- References.- 6 The Mouse Trisomies: Experimental Systems for the Study of Aneuploidy.- 1. The Generation of the Mouse Trisomies.- 1.1. Complete Trisomies.- 1.2. Partial Trisomies.- 2. Properties of the Mouse Trisomies.- 2.1. Phenotypes of the Complete Trisomies.- 2.2. Phenotypes of the Partial Trisomies.- 2.3. Determinants of the Phenotypic Expression of Aneuploidy.- 3. Mouse Trisomies As Models for Studying the Effects of Aneuploidy and Human Chromosomal Disorders.- 3.1. The Rationale of Animal Models.- 3.2. Models of Specific Human Diseases.- 3.3. Concluding Remarks.- References.- 7 Embryonic Induction and Teratology of the Developing Skin and Oral Mucosa.- 1. Introduction.- 2. Embryonic Induction.- 2.1. General Principles.- 2.2. The Role of the Extracellular Matrix (ECM).- 3. Genetic Defects and Mutants As Models for Experimental Studies.- 3.1. Genetic Defects in Human Skin Diseases.- 3.2. Animal Models of Epithelial Defects.- 4. Molecular Teratology: Virus-Induced Genetic Lesions.- 5. Summary.- References.- 8 Fine Structure of Hereditary Defects of the Central Nervous System in Mice.- 1. Introduction.- 2. Sources of Mutant Material.- 3. Methods of Study.- 4. Mouse Mutants.- 4.1. Neural Tube Defects.- 4.2. Cerebellum.- 4.3. Visual System.- 4.4. Auditory System.- 4.5. Myelination Disorders.- 4.6. Circulatory System of CNS.- 5. Other Mutants.- 6. Conclusion.- References.- 9 The Role of Mammalian Embryo Culture in Developmental Biology and Teratology.- 1. Introduction.- 2. Culture Techniques.- 2.1. Preparation of Embryos.- 2.2. Medium and Atmosphere.- 2.3. Equipment.- 2.4. Technical Proficiency.- 2.5. Maintenance of Older Embryos.- 2.6. Rat versus Mouse Embryos.- 2.7. Survival Time in Culture.- 2.8. Advantages of the System.- 3. Uses of the System.- 3.1. Developmental Studies.- 3.2. Teratology and Toxicology Studies.- 4. Disadvantages of the Culture System.- References.