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1. Fetal membranes

Fetal membranes are the accessory structures of the fetus, mainly including chorion, yolk sac, allantois, amniotic membrane and umbilical cord. It has functions such as protecting the embryo and exchanging materials with the mother's body.

The fetal membranes detach from the fetus when the fetus is delivered.

(1) Chorion

1. Development and formation The chorion develops from the trophoblast and extraembryonic mesoderm. In the second week of the embryo, the trophoblast first protrudes to the surroundings to form villi, and then extraembryonic mesoderm enters the central axis of the villi to form blood vessels. The villi contain fetal blood.

2. Classification can be divided into smooth chorion and dense chorion.

(1) Smooth chorion: It is a chorion adjacent to the decidua and the villi gradually degenerate.

(2) Dense chorion: It is a chorion adjacent to the decidua basalis and has vigorous villi development. It forms the placenta with the decidua basalis.

3. Main functions: The villi of the chorion extend into the decidua of the mother's uterus, absorb nutrients, supply the growth and development of the embryo, and at the same time discharge the embryo's metabolites.

(2) Amniotic membrane

1. Amnion and amniotic cavity The amniotic membrane is a translucent membrane. The amniotic cavity is filled with amniotic fluid, and the embryo grows and develops in the amniotic fluid.

2. Amniotic fluid Amniotic fluid is the light yellow liquid in the amniotic cavity and is secreted by the amniotic membrane.

(1) Amniotic fluid volume: Under normal circumstances, the amniotic fluid volume is 1000 to 1500ml. If it is less than 500ml, it is oligohydramnios, and if it is more than 2000ml, it is too much amniotic fluid.

Polyhydramnios or too little amniotic fluid may indicate abnormal fetal development.

(2) Functions of amniotic fluid: ① Protect the fetus from shock and extrusion by external forces; ② Prevent adhesion between the fetus and the amniotic membrane; ③ Dilate the cervix and flush the cervix during delivery

, Lubricate the birth canal.

(3) Yolk sac

The yolk sac is wrapped into the umbilical cord, eventually becoming atretic and gradually degenerating.

(4) Allantois

The allantois is a blind tube extending from the endoderm on the caudal side of the yolk sac into the body pedicle. It is wrapped by the umbilical cord and then closed.

(5) Umbilical cord

The umbilical cord is a cord-like structure formed by amniotic membrane surrounding the body pedicle, yolk sac and allantois. There are a pair of umbilical arteries and an umbilical vein in the umbilical cord.

One end of the umbilical cord is connected to the fetal umbilicus, and the other end is connected to the placenta. It is about 55cm long. The umbilical cord is the vascular channel between the fetus and the placenta, and is also the material exchange channel between the fetus and the mother.