After implantation, the endometrium is known as decidua. 

Three types of decidua:

  • Decidua basalis – forms maternal part of placenta
  • Decidua capsularis – degenerates
  • Decidua parietalis – degenerates
Subdivisions of decidua

The chorionic villi over the embryonic pole remain and develop to form numerous villi. That part is known as chorion frondosum and is the fetus part of placenta.

The remaining part of the chorionic vesicle has no villi and is known as chorion laeve.

Placenta: Chorion frondosum and decidua basalis

Placental barrier: separates fetal and maternal blood and is made of:

1. Early pregnancy

  • Capillary wall
  • Extraembryonic mesoderm
  • Cytotrophoblast
  • Syncytiotrophoblast

2. Late pregnancy

  • Capillary wall
  • Syncytiotrophoblast

Placenta increases in thickness due to villi elongation, intervillus space increases

Placenta increases in diameter due to secondary growth of uterine wall

Functions of placenta:

  1. Nutrition
  2. Respiration
  3. Excretion
  4. Protective
  • Prevents most microorganisms
  • Antibodies transmitted
  • Prevents blood from mixing

5. Secretory

  • Progesterone
  • Estrogen (make uterus sensitive to oxytocin hormone)
  • Gonadotropic hormones


1. Placenta previa – implantation occurs in lower part of uterus

Image result for placenta previa lateralis
Types of placenta previa

2. Diffuse placenta – placenta lines greater part of uterine cavity

3. Bidiscoid placenta – placenta has 2 disc like equal parts, where each receive a branch from the umbilical artery

4. Accessory placenta – placenta has accessory lobes separate from the main placenta

Accessory placenta

5. Placenta accreta, increta or percreta – placenta too deeply attached to uterus

Image result for placenta accreta, increta, percreta