The four types of placenta are diffuse, cotyledonary, zonary, and discoid. These categories describe the shape and structure of the placenta across different mammalian species, based on how much of the membrane surface actually connects to the uterine wall. Humans have a discoid placenta, but most mammals fall into one of the other three groups. Beyond shape, placentas are also classified by how deeply they invade the uterine lining, which creates a second, equally important system of categorization.
The Four Shape-Based Types
Placental shape varies dramatically across the animal kingdom. The classification comes down to how the outermost fetal membrane (the chorion) attaches to the mother’s uterus. Some species use the entire membrane surface, while others concentrate the connection into a small patch or band.
Diffuse
In a diffuse placenta, the entire surface of the fetal membrane participates in nutrient exchange with the uterus. There’s no concentrated attachment point. Instead, the connection is spread evenly across the whole membrane, like a sheet pressed flat against the uterine wall. Horses and pigs are the classic examples. This design creates a broad, shallow connection rather than a deep, focused one.
Cotyledonary
Rather than one continuous surface, a cotyledonary placenta uses dozens of small, button-like patches scattered across the membrane. Each patch, called a cotyledon, forms its own little zone of nutrient exchange. Ruminants like cows, sheep, and goats have this type. If you’ve ever seen an image of a cow’s placenta, the multiple round attachment sites are cotyledons, each one connecting to a corresponding spot on the uterine wall called a caruncle.
Zonary
A zonary placenta forms a band or belt of tissue that wraps around the middle of the fetal sac, like a stripe around a ball. This band can be complete (encircling the entire sac) or incomplete (covering only part of the circumference). Dogs, cats, and other carnivores have zonary placentas. The nutrient exchange happens only within that central band, not at the top or bottom of the sac.
Discoid
A discoid placenta concentrates all the attachment into a single, roughly circular disc on one side of the fetal membrane. This is the human type, and it’s also found in primates, rodents, and rabbits. Some species have two discs instead of one, which is called bidiscoid. The discoid design creates a compact, dense connection point where fetal and maternal blood supplies come into very close contact.
The Three Invasion-Based Types
Shape is only half the story. Placentas also differ in how deeply fetal tissue penetrates the uterine wall, which determines how many tissue layers separate the mother’s blood from the fetus’s blood. This classification matters because it directly affects how efficiently nutrients and oxygen pass between mother and offspring.
In an epitheliochorial placenta, there’s minimal invasion. The fetal tissue simply rests against the uterine lining without breaking through any layers. No maternal tissue is destroyed or removed. Pigs and horses have this type, which is the least invasive of all placental designs.
In an endotheliochorial placenta, the fetal tissue pushes deeper. The uterine surface lining and its underlying connective tissue break down after implantation, bringing the fetal cells into direct contact with the walls of the mother’s blood vessels. Dogs and cats have this intermediate type.
In a hemochorial placenta, which humans have, the invasion goes furthest. All layers of maternal tissue erode away, so the fetal tissue is bathed directly in the mother’s blood. This creates the most efficient transfer of oxygen and nutrients but also means implantation is a more aggressive process. This explains why human implantation involves the embryo essentially burrowing into the uterine lining.
How Shape and Invasion Combine
These two classification systems overlap in interesting ways. Horses have a diffuse, epitheliochorial placenta: broad surface area but shallow invasion. Dogs have a zonary, endotheliochorial placenta: a band shape with moderate invasion. Humans have a discoid, hemochorial placenta: a small attachment area but the deepest possible invasion. The combination of shape and invasion depth reflects millions of years of evolutionary tradeoffs between efficiency and safety for both mother and fetus.
A species doesn’t need a large attachment area if its placenta invades deeply enough to put fetal tissue right next to maternal blood. Conversely, a species with shallow invasion compensates by spreading the placenta across a wider surface. Both strategies get nutrients to the fetus effectively, just through different designs.
Human Placental Variations
While all human placentas are discoid and hemochorial, individual placentas can vary in structure. About 10% of women have placentas with unusual shapes, including notched, lobed, or membranous forms. Most of these variations cause no problems and are discovered incidentally during delivery or on ultrasound.
A few variations do carry clinical significance. A succenturiate placenta has one or more extra lobes connected to the main disc by blood vessels running through the membranes. It’s rare but can cause bleeding if the connecting vessels are damaged during delivery.
A circumvallate placenta has membranes that fold back on themselves around the edge of the disc, creating a raised ridge. This variant is associated with a notably higher rate of complications. In one study, women with circumvallate placentas had a 64% rate of preterm delivery, a 10.9% rate of placental abruption (where the placenta separates from the uterine wall prematurely), and a 55% rate of newborns requiring intensive care. Second-trimester vaginal bleeding combined with premature rupture of membranes can be a warning sign of this variant.
Placental position also matters. Placenta previa, where the placenta covers or sits near the cervical opening, affects 0.3% to 2% of pregnancies in the third trimester. Its incidence has been increasing alongside rising rates of cesarean delivery, since uterine scarring from prior surgeries makes abnormal placental positioning more likely.