Basic Teratology, Placenta and Fetal Membranes Orientation

This article orients you to three subjects: the general principles of teratology, the basic structure and function of the placenta, and the fetal membranes and umbilical cord. It establishes the foundational vocabulary and concepts for each.

A teratogen is any agent, chemical, infectious, physical, or environmental, capable of producing a structural or functional abnormality in a developing embryo or fetus following maternal exposure during pregnancy.

The effect of a teratogen depends on several interacting factors, not on the identity of the agent alone.

• Timing of exposure relative to the critical period of the affected structures is generally the single most important variable. The same agent can be harmless, mildly disruptive, or severely malformative depending on which developmental window exposure coincides with.
• Dose and duration matter. Most teratogens show a threshold below which no discernible harm occurs, and many show a dose-dependent relationship between exposure and severity above that threshold.
• Genetic susceptibility of both mother and embryo influences outcome, because maternal metabolism of an agent and embryonic sensitivity to it vary between individuals. The same exposure at the same dose and timing does not produce an identical outcome in every pregnancy.

Recognised categories of teratogenic agent are infectious organisms, chemical and pharmaceutical agents, physical agents such as ionising radiation, and specific maternal metabolic states. Poorly controlled maternal diabetes is a well-established example of a maternal physiological state acting as a teratogenic influence.

The placenta is the organ of exchange between mother and fetus. It forms jointly from fetal tissue (derived from the trophoblast) and maternal tissue (derived from the endometrium). It is unique among human organs in being formed from the combined contribution of two genetically distinct individuals.

The placenta transfers oxygen and nutrients from the maternal to the fetal circulation, and carbon dioxide and metabolic waste in the reverse direction. This occurs across a specialised exchange interface that keeps maternal and fetal blood in close proximity without the two circulations directly mixing under normal circumstances.

The placenta is also a major endocrine organ throughout pregnancy. It produces hormones including human chorionic gonadotrophin, progesterone, and oestrogens, which are essential for maintaining the pregnancy and preparing the maternal body for parturition and lactation.

The placenta develops from the chorionic villi, tree-like projections of trophoblastic tissue that invade the maternal endometrium and become bathed in maternal blood within the intervillous space. It grows and matures progressively throughout pregnancy alongside the fetus.

The fetal membranes are two closely apposed layers, the amnion and the chorion. Together they form the fluid-filled sac that surrounds and protects the developing embryo and fetus throughout gestation.

The amnion is the inner membrane, thin and avascular. It directly encloses the amniotic cavity and the amniotic fluid within which the embryo and fetus float and develop.

Amniotic fluid:

• Provides mechanical protection by cushioning the fetus against external pressure.
• Allows relatively unrestricted fetal movement, essential for normal musculoskeletal development.
• Is produced mainly by the fetal kidneys from around mid-gestation onward. Its volume is therefore a clinically important indirect indicator of fetal renal and urinary tract function.

The chorion is the outer membrane, formed from trophoblast together with adjacent extraembryonic mesoderm. It is the fetal tissue layer that contributes directly to formation of the placenta, unlike the purely protective, fluid-containing amnion.

The umbilical cord connects the fetus to the placenta. It forms from the connecting stalk established during embryonic folding. It is the sole physical and vascular conduit for all exchange between fetus and placenta throughout pregnancy.

The cord normally contains two umbilical arteries, carrying deoxygenated blood and waste products from the fetus toward the placenta, and one umbilical vein, carrying oxygenated, nutrient-rich blood from the placenta to the fetus.

These vessels are embedded in a specialised, gelatinous connective tissue, Wharton's jelly. It cushions the vessels against compression and kinking, which is essential because any significant, sustained compression would directly compromise fetal oxygen and nutrient supply.

A single umbilical artery, rather than the normal two, is a recognised finding on antenatal ultrasound and prompts further evaluation. It is associated with other congenital anomalies, particularly renal and cardiac.