Developmental Abnormalities, Teratogenesis and O&G Presentations
Start from one idea: normal development is an ordered sequence of steps, each happening in a narrow window, each built on the one before. A structure has to form from the right tissue, then fold, fuse, rotate, septate or canalise on schedule. An abnormality is simply one of those steps failing, or a normal structure being damaged or distorted afterwards. Everything in this chapter is a variation on that single sentence.
This reframes a long anomaly list into a small set of questions. Instead of "which abnormality do I remember?" ask: what was meant to form, when, from which tissue, and what clinical presentation follows when that step fails? A neural tube defect, a septate uterus, an imperforate hymen and a salt-wasting newborn with atypical genitalia look unrelated until you see them as four different developmental steps that did not complete. The list shrinks to a method.
Five principles organise the chapter:
- timing determines vulnerability;
- mechanism determines the pattern;
- associated anomalies often reflect shared embryological origin;
- counselling must separate diagnosis, reproductive implication and immediate safety;
- respectful language matters, especially in genital tract and sex development differences.
The Primary task is to build the mechanism. The Intermediate and Final chapters linked later take the same mechanisms into counselling, imaging, surgery and management.
Teratogenesis
The simplest cause of a failed step is an outside influence arriving while that step is happening. That is teratogenesis, so it is the natural place to begin: it makes the principle of timing concrete before we look at the structures themselves.
A teratogen is an exposure that can disturb development and produce structural, growth, functional or neurodevelopmental abnormality. The effect depends on dose, duration, route, maternal metabolism, placental transfer, fetal genotype and the tissue that is developing at the time. The word "teratology" simply names the study of these abnormalities of development, and an agent earns the label "teratogenic" only when exposure at a particular developmental stage produces a structural or functional abnormality — both timing and dose are part of the definition, not afterthoughts.
| Exposure timing | Dominant effect | Counselling logic |
|---|---|---|
| Pre-implantation | Loss or survival without a fixed structural defect | Structural anomaly risk is usually not the main issue |
| Organogenesis | Major structural malformation | Highest vulnerability for classic malformations |
| Fetal period | Growth, function, maturation, neurodevelopment | Structural risk is lower but not absent |
Timing is the most important exam principle. Before implantation, severe injury often causes loss or no lasting structural effect. During organogenesis, organs are forming, folding, fusing, septating, rotating and canalising, so exposures can produce major structural anomalies. Later in pregnancy, exposures more often affect growth, function, endocrine programming, brain development or maturation.
Types of teratogenic or fetopathic influences include:
- drugs and chemicals;
- alcohol and smoking-related toxins;
- infections;
- ionising radiation at sufficient dose;
- maternal disease such as diabetes or phenylketonuria;
- nutritional deficiency such as folate deficiency;
- environmental endocrine-disrupting substances such as selected phthalates, bisphenols, PFAS and pesticides, especially as population-level developmental and reproductive-health concerns;
- mechanical constraints such as severe oligohydramnios;
- placental dysfunction and hypoxia.
Counselling requires precision. "Exposure in pregnancy" is not enough. You need the exact substance, dose, route, duration and gestational age. You also need the background risk of congenital anomaly, because no pregnancy has zero risk.
| Determinant | Why it changes risk | Example of reasoning |
|---|---|---|
| Timing | Organs have critical windows | Neural tube closure is early, so late booking folate cannot fully prevent NTDs |
| Dose and duration | Teratogenesis often has a threshold or dose response | Repeated high exposure differs from a single minimal exposure |
| Placental transfer | Fetal dose is not always maternal dose | Small lipophilic molecules usually cross more easily than large protein-bound molecules |
| Maternal disease | The illness may be teratogenic or fetopathic | Poor periconception glycaemic control raises malformation risk |
| Genetic susceptibility | Same exposure can have different outcomes | Folate metabolism, detoxification and repair pathways modify vulnerability |
This is also why stopping necessary treatment can be dangerous. A medication may carry fetal risk, but uncontrolled epilepsy, hypertension, diabetes, tuberculosis, HIV, severe mental illness or sepsis may carry greater maternal and fetal risk. Teratology is risk balancing, not reflex avoidance.
Environmental Endocrine Disruption and Development
Classic teratology teaching often focuses on high-dose, single-agent exposures that produce visible structural malformations. Modern reproductive science also has to consider lower-dose, repeated and mixed environmental exposures that may affect endocrine signalling, placental biology, fetal growth, gametes or neurodevelopment without producing one pathognomonic anomaly.
| Exposure group | Mechanism to understand | Developmental concern |
|---|---|---|
| Phthalates | plasticiser exposure, anti-androgenic/steroidogenic and oxidative-stress pathways | reproductive tract development, ovarian function, semen quality and pregnancy outcome associations |
| Bisphenols | oestrogen-receptor and metabolic signalling interference | ovarian, endometrial, placental and fetal programming concerns |
| PFAS | persistent chemicals with immune/endocrine/metabolic associations | fetal growth, thyroid/immune and pregnancy-outcome concerns in population studies |
| Pesticides/dioxins | receptor, enzyme and epigenetic effects vary by agent | fertility, miscarriage, fetal growth and developmental programming concerns |
The counselling rule is important. These exposures are often ubiquitous and socially patterned, so the answer should not blame the patient or promise that one avoidance step removes risk. A precise Primary answer says: endocrine disruptors can mimic, block or alter hormone synthesis/metabolism/signalling; the developing embryo/fetus is vulnerable because signalling windows are time-specific; evidence often comes from epidemiology, animal models and mechanistic studies; and public-health regulation plus practical exposure reduction are the rational interventions.
This sits between teratology and public health. It explains why environmental history belongs in reproductive medicine, but also why individual causation is difficult to prove after a single adverse outcome.
The Vocabulary of Abnormality
Before naming individual anomalies, fix the vocabulary, because each term encodes a different mechanism, and the mechanism drives recurrence risk and counselling. The umbrella word is anomaly: any deviation from the expected normal structure, form or function. Underneath it sit terms that say how the deviation arose.
A malformation is an intrinsic developmental error in the formation of a structure — the developmental process itself was abnormal. Neural tube defects and Mullerian anomalies are malformations.
A dysplasia is abnormal organisation of a tissue, or defective histogenesis — the cells of a tissue assemble abnormally rather than an organ failing to form. Skeletal dysplasias and ectodermal dysplasias are examples; the defect is in how tissue is built, not in whether the organ appeared.
A deformation is an abnormal shape or position of a structure that formed normally, caused by mechanical forces. Limb positioning problems and talipes from severe oligohydramnios are deformations.