A truly unique organ

The placenta is a round, flat, temporary organ that connects the wall of the uterus to a developing fetus. It acts as an intermediary between the blood supply of the mother and that of the fetus, ensuring that their blood never mixes (mixing blood can be fatal for both the mother and the fetus if they do not share the same blood type). The placenta has three primary functions: providing the fetus with oxygen and nutrients, removing waste and producing hormones such as human chrorionic gonadotropin, progesterone and estrogen.


The placenta begins to develop in the very early stages of pregnancy. In most cases, the blastocyst (a mass of cells that will eventually become the embryo and its supporting tissues) implants in the wall of the uterus about five days after fertilization. The outer layer of cells in the blastocyst, called the trophoblast, begin to develop into the placenta. The inner cells, called the embyoblast, become the embryo.

As the blastocyst implants in the endometrium of the uterus, it breaks through the tissue and taps in to the mother's blood supply. Blood containing oxygen and nutrients begins to flow through the placental barrier from the mother's arteries to the fetus via the umbilical cord. Conversely, waste materials (such as carbon dioxide) are carried away from the fetus and deposited in the mother's blood supply for eventual disposal by her lungs and kidneys.

The placental barrier filters out many substances which can be harmful to the fetus, but it cannot filter out all toxins. Alcohol, some chemicals that enter the body via cigarette smoke and some viruses are not filtered out and can damage the developing fetus.

The placenta continues to grow throughout the duration of the pregnancy. By the end of the third week after fertilization, the placenta covers nearly a quarter of the uterus. By the end of the pregnancy, it is typically about six to eight inches in diameter and one inch thick.

During childbirth, the placenta is expelled from the uterus alongside the fetus. In most cases, this occurs within 15 to 30 minutes after the baby has been born.

Problems with the Placenta

There are several problems that can arise with the placenta. Some of the most common include:

  • Placenta Previa: Occasionally spelled placenta praevia, this problem occurs when the placenta implants low in the uterus and blocks all or part of the cervix. It is unknown what causes this problem. Symptoms include painless vaginal bleeding and pain in the lower back or belly. In many cases, the problem resolves itself as the pregnancy progresses and the uterus grows. The typical treatment in cases that do not resolve naturally is a caesarean section.
  • Placenta Accreta: This is a rare, severe problem that involves the placenta being so deeply attached to the wall of the uterus that problems may arise when it detaches (or fails to detach) during childbirth. Severe hemorrhaging can result and may endanger the mother's life. It is very difficult to diagnose placenta accreta prior to childbirth. Treatment often involves an abdominal hysterectomy.

Disposal of the Placenta

In most cases, the placenta is treated as medical waste and incinerated after childbirth. Recently, there has been a trend towards a practice called placentophagy--eating the placenta. Advocates of this practice argue that the placenta is rich in nutrition and consuming it can help to prevent postpartum depression and other medical complications. There is little, if any, medical evidence to support this position.