Placenta 101

In this section, you will find out more about the structure and function of the organ responsible for nourishing the baby and ensuring a successful pregnancy progression.


  • The major functions of the placenta are to supply oxygen and nutrients to the baby and remove waste products from the baby
  • The mother’s blood and the baby’s blood come into very close contact in the placenta but DO NOT mix
  • The placenta is attached to the mother through the wall of the uterus and is attached to the baby via the umbilical cord

What is the placenta?
How does the placenta work?
Sounds complex – give me an analogy!
Research and the placenta

What is the Placenta?

The placenta is a new organ formed in the uterus during pregnancy, creating a vital connection between the developing baby and the mother. Soon after conception, the placenta implants in the wall of the uterus and the umbilical cord arises from it. This oval, flat organ is the place where maternal and fetal blood are brought into very close contact – yet they do not mix – so that the mother can supply oxygen and nutrients to the baby and at the same time remove waste products from the baby’s blood. After birth (postnatal life), these waste products are removed by the lungs (carbon dioxide) and kidneys (urea and creatinine). The image below explains the role the umbilical cord, the placenta and the uterine artery in a health pregnancy. 


How does the placenta work?

The placenta is connected to the baby by the umbilical cord and to the mother through her uterine wall. The umbilical cord contains three vessels: two arteries and one vein. Through the umbilical arteries, the fetus pumps “used” blood with wastes and carbon dioxide into the placenta, while blood that is rich in nutrients and oxygen returns from the placenta back to the baby via the larger umbilical vein.

The mother sends blood to the placenta via her uterine arteries – these become much wider (dilated) in pregnancy to deliver more maternal blood to nourish the developing fetus. In the non-pregnant state, only 50mL/minute of maternal blood enters the uterus. By mid-pregnancy, this has increased 5-fold to 250mL/minute and on, up to over 700mL/minute at the end of pregnancy. The mother achieves this by increasing her circulating blood volume (from 5L to 8L) and her cardiac output.

The normal placenta is long and thin, and inside contains lots of small placental villi, arranged as dangling bunches of grapes. In the diagram above, the mother’s blood comes up each side (right and left) of the uterus in the widely dilated uterine arteries (red). In this diagram, a total of 7 branches are drawn, representing the decidual branches of the uterine arteries that direct blood to the villi. The combination of a placental villous tree and its feeding decidual artery is called a placentome. There are about 50 of these individual functional units stuck together to form the placenta.

This diagram will be modified in other parts of the website in order to explain Doppler ultrasound and placental insufficiency.

Sounds complex: give me an analogy!

A simple but effective analogy of placental structure is to imagine that you have several bunches of grapes. You then individually placed these bunches, stalks up, into your bath tub, and then filled the bath up with water. Imagine then that small pieces of string connected each stalk together, and these strings met in the middle of the tub and were woven into a rope.


The rope represents the umbilical cord coming out of the middle of the placenta. If the bath tap is kept on and the drain is open, then the water (representing maternal blood) flows around the bunches of grapes and out the drain continuously. This flow represents the uterine artery blood flow – the only difference is that maternal blood is actually pumped into the placenta by the actions of the mother’s heart.

If the water is warm (representing oxygen), the grapes get warmed up. The rope then gets warmed, and so “warmth” (or oxygenated blood), will go up the umbilical cord to the developing baby.

The bunches of grapes are the equivalent of the placental cotyledons (also known as placentomes). The individual grapes represent the “business end” of the placenta and are called the placental villi. The skin of the grapes keeps the grape juice in place. If the grape juice represents the fetal blood, then the grape skin represents the very thin placental barrier between the mother’s blood circulation and the baby’s blood circulation. This placental layer is deliberately very thin at the tips of the villi (the terminal villi) so that the gases and metabolic waste products from the baby can pass freely to maternal blood to be removed by her lungs and kidneys.

Research and the Placenta

The human placenta shares about 95% of its genetic code with small rodents, such as mice. This is important because it is now possible to manipulate the genetic code in mice and breed them (transgenic mice) to see which genes are critical for mammalian placental development. The Lunenfeld-Tanenbaum Research Institute at Mount Sinai Hospital is a world leader in advancing our understanding of both human and mouse placental development.

Whilst many individual aspects of placental function (such as nutrient transport, oxygen transfer, blood flow regulation) can be studied in the laboratory, only a few components can be measured “risk-free” or non-invasively in a real-time clinical setting.

Special Pregnancy Program

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Fax: 416-586-5109

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Prenatal Diagnosis & Medical Genetics

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phone: 416-586-4800 x 4523
fax numbers:
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Perinatal Mental Health

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Phone: 416-586-4800 x 8325
Fax: 416-586-8596