Placental Development

Twinning

Dizygotic (fraternal) twins develop from two zygotes

The fetuses have two placentas, two chorions, and two amniotic sacs

Monozygotic (identical) twins develop from one zygote

Sixty-five percent of cases have one placenta, one chorion, and two amniotic sacs

Thirty-five percent of cases have two placentas, two chorions, and two amniotic sacs

The placenta is the primary site of nutrient and gas exchange between the mother and fetus

IgG also crosses the placental membrane, conferring passive immunity

The maternal component is a portion of the endometrium called decidua basalis

Decidual cells secrete prostaglandins E2 which inhibits T lymphocyte activation

The fetal component consists of villous chorion

Cytotrophoblast composes the inner layer of the villous chorion (fetal component of the placenta)

Syncytiotrophoblast is the outer layer of the placenta

Secretes hCG and lacks major histocompatibility complex antigens → doesn’t invoke an immune response