Placenta and Fetal Membranes

Embryonic Development

• By the end of the third week, a primordial uteroplacental circulation has developed.
• The embryo, amniotic sac, and umbilical vesicle are suspended in the chorionic cavity by the connecting stalk.

Chorionic Villi Development

• Secondary chorionic villi develop into tertiary chorionic villi.
• The process is illustrated in figures A, B, C, and D, which show the development of the chorionic villi from 14 to 21 days.

Yolk Sac and Allantois

• The yolk sac is formed from the hypoblast and is surrounded by the connecting stalk.
• The allantois is an endodermal outpocketing of the yolk sac that invades the connecting stalk.
• Blood cell formation occurs in the allantois during the third to fifth weeks.
• The allantois eventually becomes the umbilical vessels and the urachus ligament.

Placenta Development

• The placenta is a fetomaternal organ with two components: a fetal part from the chorionic sac and a maternal part from the endometrium.
• The placenta functions in protection, nutrition, respiration, excretion of waste products, and hormone production.

Amniotic Sac and Membrane

• The amniotic sac enlarges faster than the chorionic sac, causing the amnion and smooth chorion to fuse and form the amniochorionic membrane.
• The amniochorionic membrane ruptures during labor, and preterm rupture can lead to premature labor.

Placental Circulation

• The placental membrane consists of four layers: syncytiotrophoblast, cytotrophoblast, placenta, and fetal membranes.
• There are two types of circulation: fetoplacental circulation and maternal-placental circulation.

Decidua Development

• The decidua is the mucous membrane lining the uterus, and it undergoes a transformation (decidual reaction) due to cyclic adenosine monophosphate and progesterone signaling.
• The decidual reaction provides an immunologically privileged site for the conceptus.

Placental Functions

• The placenta functions in metabolism, transport of gases and nutrients, and excretion of waste products.
• The placenta also produces hormones, such as human chorionic gonadotropin (hCG), human chorionic somatomammotropin (placental lactogen), human chorionic thyrotropin, and human chorionic corticotropin.

Amniotic Fluid Significance

• Amniotic fluid permits uniform external growth of the embryo, acts as a barrier to infection, and prevents adherence of the amnion to the embryo.
• It also cushions the embryo against injuries, controls embryonic body temperature, and enables fetal lung development.

Parturition

• The process of childbirth includes dilation, expulsion, and placental separation.

Multiple Pregnancies

• Dizygotic (DZ) twins develop from two zygotes and have two amnions and two chorions, while monozygotic (MZ) twins develop from one zygote and are genetically identical.