Teratology and Congenital Malformations

1. Congenital Anomaly Is a Description
2. Malformation, Deformation, Disruption and Sequence
3. Teratology Is a Timing Problem
4. Critical Periods from Conception to Birth
5. Placental Transfer and Drug Patterns
6. Infection, Maternal Disease and Environment
7. Prevention before Organogenesis
8. Screening and Diagnostic Detection
9. References and Further Reading
10. Quiz

• Congenital means present from before birth (prenatal life); it does not specify genetic cause or developmental mechanism.
• Malformation, deformation and disruption describe different initiating processes, while a sequence is a downstream cascade from one primary anomaly.
• The outcome of exposure to a teratogen (an agent that disturbs normal development) depends on developmental timing, dose and duration, access to the conceptus (the developing embryo or fetus with its membranes) and maternal and fetal susceptibility.
• Early loss, major structural change, altered growth and functional impairment occupy different but overlapping windows.
• Screening estimates chance or detects features; diagnostic testing answers a defined question in sampled material and still has limits of scope.

Congenital Anomaly Is a Description

A congenital anomaly is a structural or functional difference that originates before birth. It may be recognised on antenatal (before-birth) imaging, at delivery or only later, as with some hearing, visual or metabolic effects. The word congenital describes timing of origin, not cause. A congenital condition can arise from a chromosome or gene change, an environmental exposure, an infection, maternal disease, mechanical constraint, or an interaction among several factors.

Teratology is the study of abnormal development produced by environmental conditions or agents. A teratogen is such an agent, so it is not simply anything encountered during pregnancy, and exposure is not equivalent to injury. The agent must reach a susceptible tissue at a biologically relevant dose and time. Many exposed pregnancies are unaffected, while the same agent can produce different outcomes at different stages. Conversely, a recognised anomaly does not identify its cause from appearance alone.

This lesson supplies a mechanism-and-timing framework rather than a list of syndromes. Normal limb and body-wall patterning is taught in Development of the Limbs and Body Wall. Here the first question is different: did tissue form abnormally, become distorted, break down after normal formation, or trigger a cascade of secondary effects?

Malformation, Deformation, Disruption and Sequence

A malformation arises from an intrinsically abnormal developmental process during formation of a structure. The organ may be absent, incomplete or patterned abnormally. Genetic and environmental causes can both disturb intrinsic morphogenesis (the shaping of tissues and organs), so malformation does not mean inherited. A deformation changes the shape or position of a part that initially formed normally, usually through mechanical constraint. Reduced amniotic fluid (the fluid around the fetus) or limited uterine space can distort limbs even though their bones were patterned correctly.

A disruption is extrinsic breakdown of, or interference with, tissue that had been developing normally. Amniotic bands (strands of the membrane around the fetus) can constrict and damage a previously formed limb, producing an irregular distribution unlike a repeatable patterning defect. A sequence differs from these three: it is not a further kind of tissue injury but a chain of secondary anomalies caused by one initiating malformation, deformation or disruption. Dysplasia (abnormal organisation of tissue, which most commonly affects skin, brain, cartilage or bone) is a further mechanism that this lesson does not cover.

The oligohydramnios sequence (the pattern that follows too little amniotic fluid) shows the logic. Severe renal malformation can reduce fetal urine and therefore amniotic fluid. Low fluid then permits compression-related limb and facial deformation and restricts lung expansion, producing pulmonary hypoplasia (underdeveloped lungs). The later findings are not independent renal malformations. Reading the chain backwards identifies one primary event. This stops a sequence being confused with a syndrome, in which several features share an underlying cause but are not simply downstream of one initiating anomaly.

Classify the initiating event first: abnormal formation, altered shape, tissue breakdown, or downstream cascade.

SimpleMed original educational diagram

Teratology Is a Timing Problem

Core teratology principles turn an exposure history into a developmental question. Susceptibility depends on the maternal and conceptus (embryo or fetus and its membranes) genotypes, developmental stage, dose and duration, route and placental access, and the agent's mechanism. Outcomes include embryonic or fetal death, structural malformation, altered growth and functional impairment. These are not mutually exclusive, and risk is rarely described by dose alone.

During roughly the first two post-fertilisation weeks, before and around implantation, severe injury often causes loss while a surviving conceptus may develop without a major structural anomaly. This is the useful all-or-none rule of thumb: severe injury tends to cause loss, while a survivor tends to be structurally normal. A small pool of relatively unspecialised cells can sometimes compensate, whereas extensive injury cannot. It is not a guarantee. Genetic damage, mosaicism (cells with different genetic make-up in the same person) or an agent persisting into later development can escape the simple rule.

Once cell populations are committed to particular organs, timing becomes anatomically predictive. Interference while a heart septum, limb field (the group of cells that forms a limb) or palate (the roof of the mouth) is being constructed can alter structure. The same exposure after gross form is established may instead affect growth, renal function, brain maturation or hearing. Timing often changes the phenotype more dramatically than the agent's name.

The exposure may be unchanged; the developmental task underway redirects the likely outcome.

SimpleMed original educational diagram

Critical Periods from Conception to Birth

The embryonic period of organogenesis (the formation of the organs), approximately post-fertilisation weeks three to eight, carries the broadest risk of major structural malformation. Organs do not develop simultaneously. The heart begins major morphogenesis early, limb patterning occupies a narrower later span, and palate formation continues after early facial patterning. A calendar week is therefore meaningful only when paired with the structure and process active then.

From post-fertilisation week nine to birth, the fetal period is dominated by growth and functional maturation, but vulnerability does not end. Brain organisation, neuronal migration (nerve cells moving to their final positions), synapse formation, eye and ear function, renal physiology and genital maturation continue. Structural effects can still occur where morphogenesis persists, while growth restriction and functional impairment become relatively more prominent. A normal external form cannot prove normal later function.

Clinical pregnancy dating usually counts from the last menstrual period and is about two weeks ahead of post-fertilisation age. The diagram uses post-fertilisation age to match developmental events. Its bands are deliberately broad: critical periods overlap, individual development varies, and colour boundaries are not safe-versus-dangerous switches.

Critical periods overlap. Rust marks peak structural vulnerability; gold marks continuing growth or functional vulnerability.

SimpleMed original educational diagram

Placental Transfer and Drug Patterns

The placenta is an active transport and metabolic interface, not an impermeable shield. Many small, lipid-soluble, relatively un-ionised and unbound molecules cross by diffusion; transport proteins can move other compounds towards or away from fetal blood. Transfer also changes with maternal concentration, placental blood flow and gestational age (the length of the pregnancy, counted from the last menstrual period). Detectable transfer proves exposure, not teratogenicity. Harm still requires a susceptible target, relevant timing and sufficient effective dose.

Selected medicines illustrate mechanism and timing. Isotretinoin (a vitamin A-related medicine used for severe acne) can disturb retinoid signalling (the vitamin A pathway that guides development) during organogenesis and produce a characteristic craniofacial (face and skull), cardiac (heart), thymic (thymus) and central nervous system pattern. Valproate (a medicine used for epilepsy and bipolar disorder) exposure is associated with major malformations including neural tube defects (the spine or skull failing to close properly, as in spina bifida) and with later neurodevelopmental effects (problems with learning, speech and behaviour), so its risk is not confined to one visible anatomical window. In the UK, valproate must not be prescribed to any woman or girl able to have children unless the conditions of the Pregnancy Prevention Programme are followed. Patients of either sex under 55 should only be started on it when two specialists agree that other treatments have not worked and the benefits outweigh the risks (MHRA).

Angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs) are two classes of blood pressure-lowering medicine (antihypertensives). They should be avoided in pregnancy. The BNF (British National Formulary) states that a risk of teratogenicity in the first trimester (roughly the first 12 weeks) cannot be excluded, and that fetotoxicity (harm to the fetus) is a known risk in the second and third trimesters. A woman planning a pregnancy should be switched to an alternative treatment with an established safety profile in pregnancy, unless continued treatment is considered essential (BNF). If a woman taking one becomes pregnant, NICE NG133 advises stopping it, preferably within 2 working days of notification of pregnancy, and offering an alternative. Separately from that general advice to avoid them, these drugs produce a characteristic fetopathy (damage to the fetus) that reflects the fetal renin-angiotensin system (a hormone system that helps control blood pressure and kidney function) becoming functionally important from the second trimester (roughly weeks 13 to 27) onwards. They impair fetal renal function and reduce amniotic fluid, and the resulting oligohydramnios (too little amniotic fluid) generates secondary pulmonary hypoplasia (underdeveloped lungs), skull ossification defects (poorly formed skull bone, called hypocalvaria) and limb contractures (joints fixed in a bent position). The result is fetopathy and sequence logic, not a classic early patterning defect. These examples teach principles, not prescribing. Starting, stopping or switching treatment during pregnancy requires current specialist risk-benefit guidance because uncontrolled maternal disease and abrupt withdrawal can also cause harm.

Preview the Teratology and Congenital Malformations quiz