1 Teratogenesis & twins Dr. Lubna Nazli. 2 Objectives Congenital malformations Causes & types Twins: causes and types.
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1
Teratogenesis & twins
Dr. Lubna Nazli
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Objectives
• Congenital malformations
• Causes & types
• Twins: causes and types.
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Teratology
• Teratology– Science that studies the causes of abnormal
development– Birth defects is the number one cause of
infant mortality
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Congenital Malformations• Causes
– Genetic/chromosomal– Environmental
• Incidence– 2-3% of newborn (4-6% by age 5)– In 40-60% of all birth defects cause is unknown
• Genetic/chromosomal– 10%-15%
• Environmental– 10%
• Multifactorial (genetic & environmental)– 20%-25%
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Types of Anomalies• Malformations
– Occur during formation of structures• Complete or partial absence• Alterations of its normal configuration
• Disruptions– Morphological alterations of structures after
formation• Due to destructive processes
– Vascular accidents bowel atresias
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• Deformations– Due to mechanical forces that mould a part of
fetus over a prolonged period of time• Clubfeet due to compression in the amniotic cavity• Often involve the musculoskeletal system & may
be reversible postnatally
• Syndromes– Group of anomalies occuring together with a
specific common etiology• Diagnosis made & risk of recurrence is known
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Environmental factors
• Infectious agents
• Radiation
• Chemical Agents
• Hormones
• Maternal Disease
• Nutritional Deficiencies
• Hypoxia
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Infectious Agents• Rubella (German Measles)• Cytomegalovirus• Varicella (chickenpox)• HIV/AIDS• Herpes Simplex Virus
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Radiation• Teratogenic effect of ionizing radiation well
established– Microcephaly– Skull defects– Spina bifida– Blindness cleft palate– Extremity defects
• Direct effects on fetus or indirect effects on germ cells
• May effect succeeding generations• Avoid X-raying pregnant women
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Chemical agents/Drugs
• Thalidomide– Found to cause amelia & meromelia
• Total or partial absence of the extremities
• Anticonvulsants (to treat epilepsy)• Antianxiety drugs (minor tranquilizers)• Antipsychotic drugs (major tranquilizers)• Antihypertensive agents• Aspirin
– Potentially harmful in large doses
Wednesday, April 19, 2023
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Unilateral phocomelia upper limb(Thalidomide toxicity)
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Alcohol• Relationship between alcohol consumption
& congenital abnormalities• Fetal alcohol syndrome
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Cigarette Smoking
• Has not been linked to major birth defects– Smoking does contribute to intrauterine
growth retardation & premature delivery– Some evidence that is causes behavioral
disturbances
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Hormones• Androgenic Agents
– Synthetic progestins were used frequently to prevent abortion
» Masculinization of female genitalia
• Oral Contraceptives– Low teratogenic potential, discontinue if
pregnancy suspected
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Maternal Disease• Disturbances in CHO metabolism (diabetic
mothers)– High incidence of stillbirth, neonatal deaths– Abnormally large infants– Congenital malformations– Oral hypoglycemic agents maybe
teratogenic
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Environmental Chemicals
• Mercury– Fish, seed corn sprayed with mercury
containing fungicide• Multiple neurological symptoms
• Lead abortions– Growth retardation– Neurological disorders
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Prevention of birth defects
• Good prenatal care
• Iodine supplementation eliminates mental retardation & bone deformities– Prevent cretinism
• Folate/Folic Acid supplementation incidence of neural tube defects
• Avoidance of alcohol & other drugs during all stages of pregnancy incidence of birth defects
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Principles of teratology
• Susceptibility varies with developmental stage at time of exposure– Most sensitive period for inducing birth defect is weeks 3-
8 of gestation
• Manifestations of abnormal development depend on dose & duration of exposure
• Teratogens act in specific ways on developing cells & tissues to initiate abnormal embryogenesis
• Manifestations of abnormal development death, malformation, growth retardation, functional disorders
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Chromosomal & Genetic Factors
• Numerical Abnormalities– Trisomy 21 (Down syndrome)– Trisomy 18– Trisomy 13– Klinefelter Syndrome– Turner Syndrome– Triple X Syndrome
• Structural Abnormalities
• Mutant Genes
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Twinning
Types of twins
• Monozygotic or maternal twins
• Dizygotic or Fraternal twins
• Conjoined or Siamese twins
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Human twins are classified into two major groups:
• Monozygotic (one-egg, or identical) twins and Dizygotic (two-egg, or fraternal) twins.
• Fraternal twins are the result of two separate fertilization events, whereas identical twins are formed from a single embryo whose cells somehow dissociated from one another.
• Identical twins may be produced by the separation of early blastomeres, or even by the separation of the inner cell mass into two regions within the same blastocyst
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Identical twins formationmonozygotic twins
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Conjoined Twins
• are identical twins who develop with a single placenta from a single fertilized ovum.
• are always of the same sex. • are more often females than males, at a ratio of
3:1. • occur as often as once in every 40,000 births
but only once in every 200,000 live births. • are more likely to occur in India or Africa than in
China or the United States.
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Types of Conjoined Twins
• Craniopagus: Cranial union only, about 2% of all conjoined twins.
• Pygopagus: Posterior union of the rump, about 19% of all conjoined twins.
• Thoracopagus: Anterior union of the upper half of the trunk. The most common form of conjoined twins (about 35%), it always involves sharing the heart.
• Cephalopagus: Anterior union of the upper half of the body with two faces on opposite sides of a conjoined head. Extremely rare. The heart is sometimes involved. A combination of types 3 and 4 is called cephalothoracopagus.
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• Parapagus:(sometimes called diprosopus): lateral union of the lower half, extending variable distances upward, about 5% of all conjoined twins. Heart sometimes involved.
• Ischopagus: Anterior union of the lower half of the body, about 6% of all conjoined twins. Heart not involved.
• Omphalopagus: Anterior union of the midtrunk, about 30% of conjoined twins.
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Craniopagus
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