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Placenta - Early and Later Developmental Stages

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The placenta is the first organ formed during pregnancy to aid the fetus. It has numerous immune, physiological, and endocrine functions.

Medically reviewed by

Dr. Arjun Chaudhari

Published At February 10, 2023
Reviewed AtFebruary 10, 2023


The placenta is slowly formed during the first trimester of pregnancy and grows parallel to the uterine by the fourth month. Once the development of the placenta is complete, it almost resembles a spongy disc of dimensions 3 cm in thickness and 20 cm in diameter. Several layers of tissue are present in the placenta, which has to be normally developed for the organ to function properly during gestation. Without this organ's proper functioning, the pregnancy can have devastating effects.

What Is a Placenta, and When Does It Form?

A placenta is a temporary and vital organ formed right after fertilization. It consists of the chorion, amnion, umbilical cord, and parenchyma. It gets attached to the uterine wall and connects the fetus to the uterus through an umbilical cord. The genetic characteristics of a placenta are similar to a developing fetus.

Where Is the Placenta Located?

After fertilization, the fertilized egg attaches itself to the uterine wall, and the placenta forms and attaches itself to that area. Some of the placental positions are:

  • Anterior Placenta - The placenta attaches itself to the uterine front wall, close to the abdomen.

  • Posterior Placenta - Here, the placenta attaches to the back wall of the uterus, near the spinal region.

  • Lateral Placenta - It grows on the left or right uterine wall.

  • Fundal Placenta - The placenta forms on the top region of the uterus (between fallopian tube openings called the fundus).

  • Low-Lying Placenta - The placenta lies very low in the uterus, near or covering part of the internal cervical os (a part of the cervix that opens into the uterus). Here the placenta lies less than 20 mm from the cervical region.

  • Placenta Previa - The placenta extends over the internal cervical os and covers the cervix region completely. This placental position is known as placenta previa.

What Does the Placenta Do?

The placenta is essential to the fetus and the mother's well-being. It is a channel between the mother and the fetus. It produces hormones essential for fetal growth, sustains the baby with essential nutrients and oxygen, and filters out carbon dioxide and harmful waste from the fetus. It also protects the fetus and transfers immunity from the mother to the fetus.

What Are the Early and Later Development Stages of the Placenta?

Early Development Stage

  • Pre-implantation - After conception and fertilization, the outer mass of the cell develops into the trophoblast and the inner cell mass into the embryoblast. The inner mass cells develop into the fetus and its membrane. The trophoblast develops into the placenta. The ovum that is fertilized becomes a morula. The morula forms a blastocyst by taking in the fluid, with trophoblasts surrounding the fluid and inner cell mass.

  • Implantation - The blastocyst formed by the morula gets embedded into the uterine wall, and the placenta and fetus start developing. The fertilized ovum usually gets implanted on the seventh to the tenth day. Implantation usually occurs either in the front wall or the back uterine wall. In other cases, it might even get implanted in areas like the fundus or cervix. During this time, the uterus has an environment with deficient oxygen levels. When trophoblasts come in contact with the uterine endometrium, syncytiotrophoblast and cytotrophoblast develop. Syncytiotrophoblast secretes human chorionic gonadotrophic hormone (hCG), and cytotrophoblast secretes enzymes that break down the endometrial wall, breaching it, thereby allowing the syncytiotrophoblast to invade the uterine cells of the mother.

Post Implantation - A bi-layered chorion membrane is formed with the trophoblast ectoderm (syncytiotrophoblast and cytotrophoblast) and the extraembryonic mesoderm. The chorion surrounds the fetus and forms the placenta.

  • Primary Chorionic Villi - Finger-like projections of the cytotrophoblast then grow into the syncytiotrophoblast and form the primary chorionic villi.

  • Secondary Villi - The extraembryonic mesoderm divides into splanchnic mesoderm and somatic mesoderm. The somatic mesoderm grows into the primary chorionic villi and forms the secondary villi.

  • Tertiary Villi - In the embryonic mesoderm of the secondary villi, embryonic vessels develop, forming the tertiary chorionic villi. These capillary beds connect to the embryo's heart. The villis continue to grow and branch into the fetal placenta called the villus chorion. As the development progresses, cytotrophoblast cells continue to grow in the syncytiotrophoblast forming a cytotrophoblastic shell. The maternal part of the implantation process comes from the endometrial layer called the decidua. There are three parts, namely decidua capsularis (it covers the site of implantation), decidua parietalis (it covers everything else), and decidua basalis (present deep at the site of implantation). As the amniotic sac develops, the decidua capsularis undergoes degeneration, fuses with the decidua parietalis, and eventually disappears. The amniotic sac comes in contact with the amniochorionic membrane that eventually fuses with the decidua parietalis. A chorionic plate is formed with an amniochorionic membrane and fetal vessels. A part of decidua basalis grows into the chorionic plate dividing them into different septa containing stem villi. The villi attached to the decidua basalis act as an anchor between the endometrium and the fetal sac. Through openings in the cytotrophoblastic shell, the endometrial blood vessels, known as the spiral arteries, make their way into the chorionic villi. While inside the villi, these vessels release the maternal blood into each septum of the chorionic plate. In this way, the fetus present across the placental membrane receives nutrients and oxygen from the maternal blood. The placental membrane is formed by the fetal blood vessel endothelium, cytotrophoblast, syncytiotrophoblast, and embryonic connective tissue. It is the main barrier where the fetus and the mother exchange nutrients, hormones, etc.

The umbilical cord consists of one umbilical vein and two umbilical arteries and attaches the fetus to the placenta. Fetal circulation mainly occurs from the fetus to the umbilical arteries, then to the chorionic arteries in the septa ending in the capillary beds where gas is exchanged with the maternal blood and travels back to the fetus through the umbilical vein.


The placenta is a fetal organ, and its complete development is essential for a healthy pregnancy for both the fetus and the mother. Disordered or incomplete placenta development can cause abnormalities in the fetus and affect the mother's health.

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Dr. Arjun Chaudhari
Dr. Arjun Chaudhari

Obstetrics and Gynecology


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