Teratology and Critical Periods
On this page
Direct answer
Teratology is the study of congenital malformations and their causes — genetic factors, infections, drugs, chemicals and physical agents. Susceptibility to a teratogen depends on the stage of development: in the first two weeks an insult follows the all-or-none rule, weeks three to eight, the period of organogenesis, are the most dangerous, and after week eight damage is largely functional or growth-restricted rather than structural. Knowing which agent damages which system in which window is a favourite MCQ exercise.
What you must remember
- Critical periods: pre-differentiation (weeks 1 to 2) — embryo either dies or recovers fully; organogenesis (weeks 3 to 8) — peak susceptibility to structural defects, with each organ having its own window; foetal period (weeks 9 to 38) — vulnerability of the brain, teeth and external genitalia, plus growth and functional defects.
- TORCH infections: Toxoplasma (chorioretinitis, hydrocephalus, calcification), Rubella (cataract, deafness, cardiac defects), Cytomegalovirus (microcephaly, periventricular calcification), Herpes and others including syphilis and varicella.
- Classical teratogenic drugs: thalidomide — phocomelia and limb defects; isotretinoin — retinoid embryopathy with ear, cardiac and thymic defects; warfarin — nasal hypoplasia and stippled epiphyses; sodium valproate — neural tube defects; phenytoin — foetal hydantoin syndrome; lithium — Ebstein anomaly; tetracycline — discoloured teeth; aminoglycosides — deafness.
- Maternal states: poorly controlled diabetes (macrosomia, cardiac and neural defects), iodine deficiency (cretinism), alcohol (foetal alcohol spectrum disorder with microcephaly, short palpebral fissures and smooth philtrum).
- Types of defect: malformation (poor formation of tissue), disruption (breakdown of normal tissue, as by amniotic bands), deformation (mechanical moulding, as in oligohydramnios), and dysplasia (abnormal organisation of cells, as in achondroplasia).
- Terminology: a sequence is a cascade from one defect, a syndrome is a recognised pattern with one cause, an association is a non-random grouping such as VACTERL (vertebral, anorectal, cardiac, tracheo-oesophageal, renal and limb anomalies).
- Physical agents: ionising radiation, hyperthermia in early pregnancy (neural tube defects) and maternal smoking (growth restriction).
Common confusion
Students quote the most teratogenic period as the whole first trimester; strictly it is weeks three to eight — the embryo is already two weeks old at the first missed period, so weeks four to ten after the last menstrual period are the danger zone. Also distinguish causation: Down syndrome is a chromosomal malformation, thalidomide phocomelia a drug-induced malformation, and Potter facies a deformation sequence from oligohydramnios — the mechanism dictates recurrence risk.
Exam-focused takeaway
Theory questions pair a teratogen with its signature defect — rubella with the triad, thalidomide with phocomelia, valproate with spina bifida, warfarin with stippled epiphyses. In the viva, state the critical periods, define malformation, disruption, deformation and dysplasia, and outline prevention with folate and pre-pregnancy counselling on teratogenic drugs.
Frequently asked questions
Which period of development is most sensitive to teratogens?
Weeks three to eight after fertilisation, the period of organogenesis, when the organs are being formed; each organ is most vulnerable during its own differentiation window.
What is the classical triad of congenital rubella?
Cataract, sensorineural deafness and congenital heart disease, especially patent ductus arteriosus; the risk is greatest when infection occurs in the first trimester.
Which drug caused phocomelia?
Thalidomide, prescribed as a sedative in the late 1950s, produced severe limb reduction defects when taken in weeks four to eight of gestation.
How do malformation and deformation differ?
A malformation is an intrinsic fault in tissue formation, whereas a deformation is an alteration of normally formed parts by extrinsic mechanical forces, such as uterine constraint or oligohydramnios.
What is the VACTERL association?
A non-random grouping of Vertebral, Anorectal, Cardiac, Tracheo-oesophageal fistula, Renal and Limb anomalies occurring together more often than chance, without a single defined cause.