Definition

The placenta is a temporal vascular inelastic organ of the fetus which acts as a center of vital communication between the maternal blood and fetal blood during intra-uterine life (a round flat mass).

General Characteristics

• Size: About $20\text{cm}$ in diameter and $2.5\text{cm}$ in thickness at its center.

• Weight: It weighs approximately $1/6$ of the baby’s weight at term, though this may be affected by the time the cord is clamped due to varying amounts of blood retained in the vessels.

• Situation: It is usually/normally located in the upper uterine segment. During the $3^{rd}$ stage of Labor it is expelled after separation.

• Shape: Circular flat mass.

Structure: Two Surfaces

The placenta has two distinct surfaces: the maternal surface and the fetal surface.

1. Maternal Surface

This surface lies next to the uterine wall and is dark red in color because the basal decidua separates with it.

• Cotyledons: The chorionic villi are separated into $16-20$ lobes known as cotyledons.

• Septa: These cotyledons are separated by sluices or furrows into which the decidua dips down to form septa (walls). Each cotyledon contains a single villus with its branches.