What is a placenta? The unsung hero of pregnancy


Every parent stumbles upon this question one time or another—what and how do I feed my newborn?

Natural or artificial nipples? Mashed yams or Blueberry-Apple-Spinach Puree? Cheerios or cut-up fries?

You will spend hours finding “organic only” ingredients. These are significant decisions, after all.

What your baby feeds on will shape the way your baby grows.

Here’s something that will surprise you.

Your baby has been feeding long before you started thinking about your baby’s nutrition.

I know what you’re thinking.

Hah! That’s not me. I was reading up on what to feed my baby in the first trimester itself.

What if I told you that was still late by about three months or so?

The right time to think about your baby’s nutrition is when you are trying to conceive!

Sounds exaggerated? Your baby’s first meal starts in the womb!

What is a placenta?

A placenta is an organ that develops in the uterus during pregnancy. It serves as a connection between the developing fetus and the mother.

The placenta plays a vital role in providing oxygen, nutrients, and immune support to the growing fetus while also removing waste products from the fetal blood.

It acts as a barrier, preventing the mother’s blood from mixing with the fetus’s blood but allowing essential substances to pass through, such as oxygen and nutrients.

The placenta also produces hormones that help maintain pregnancy and prepare the mother’s body for childbirth. After birth, the placenta is expelled from the mother’s body, typically referred to as the afterbirth.

What does the placenta do?

The placenta is made from fetal cells and is the first organ any mammal ever makes.

Did you know?
A week after conception, an organ starts developing in your uterus: The Placenta.

Placenta feeds baby in womb. It's fully developed by 12 weeks.

At 12 weeks, the placenta is fully developed and takes the form of a disk. That’s why the placenta derives its name after the Greek word plakos, which means “flat cake” because of its appearance.

A healthy placenta is a foundation for a healthy pregnancy and baby.

This unusual organ has two critical roles:

  1. Deliver everything your baby needs to grow—glucose, vitamins, calcium, oxygen, etc. It delivers a laundry list of antibodies too.
  2. Clear up all the waste your baby creates—things like carbon dioxide, urine, and metabolic wastes.

The placenta is called ‘The Tree of Life’

Roots grow first from the seed, before any other part.

Placenta attaches to the uterus like roots of a plant.
The placenta attaches to the uterus like roots

The placenta attaches to the uterus like roots.

The roots attach to mother earth to hold the ground firmly and absorb nutrients and water. The seed-root-earth analogy is a perfect way to visualize a baby-placenta-mother relation.

One side of the placenta attaches to the baby via an umbilical cord. The other side is densely branched and connects to the uterine wall, just like the roots of a plant.

The placenta is where you are most literally bonded to your baby!

The evolution of the placenta

A life-changing adaptation unfolded millions of years ago. Some creatures stopped laying eggs and instead gave birth to live young ones.

These creatures were the progenitors of nearly all modern mammals. The humble placenta was the key evolutionary innovation that made live birth possible. It is the reason behind the rise of mammals on Earth!

Did you know?
Fossil evidence shows that the first placental mammals evolved between 163 million and 157 million years ago (during the Jurassic Period).

Instead of laying eggs and leaving them to the mercy of predators, the mother’s womb was safer. It provided a more controlled environment for the development of the embryo.

What’s so special about the placenta?

The placenta is an organ that ensures that blood from the mother and the fetus never mix. Nutrients and oxygen pass over exclusively by diffusion.

The placenta is unlike any other organ. It connects two genetically distinct organisms yet separates them.

If the mother’s and fetus’s blood mixed, it could be deadly for both. For one, if incompatible blood groups mix, the body’s immune system reacts by forming antibodies to destroy the new blood cells.

How does the placenta work?

A placenta consists of a large mass of blood vessels from both the mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. The mother’s and fetus’s blood can exchange substances across their capillary walls without the blood mixing.

Placenta transfers nutrients without direct blood to blood contact.
The placenta transfers nutrients without direct blood-to-blood contact.

The hormones in you stimulate the inside layer of your uterus to grow the blood vessel larger, facilitating effective exchange. 

Your blood pressure forces the blood around the placenta so that nutrients and antibodies for immunity pass to your baby. 

At the same time, the baby’s waste products pass to your circulatory system for disposal. 

An umbilical cord contains two arteries and a vein connecting the fetus to the placenta. They shuttle the blood between the baby and the placenta.

Did you know?
Despite the small size of the placenta, the surface area available for the exchange of nutrients and oxygen is approximately the size of a one-bedroom house!

Your baby manipulates you through the placenta!

The placenta is your baby’s first organ.

That’s right. The placenta is not a maternal organ.

But placenta reprograms the mother’s physiology to help in developing the fetus. The placenta serves as a central regulator for the mother and baby.

It controls metabolism and hormone secretion that will help develop your baby. It also preps the mother for childbirth and childcare.

Mother in different moods: nausea, food cravings and exhaustion.
You are on a roller coaster ride, Mama!

That nausea you experience, the sudden urge to eat something sour, the mood swings that you undergo, the weird combination of foods that you crave, and your urge to take a nap whenever you get a chance—all this is your baby getting its way!

Did you know?
You would have got compliments from your friends—“I see the pregnancy glow!”—it is the estrogen hormone at work. And it’s the placenta that secretes estrogen.

Hormone changes during pregnancy

Your baby manipulates you through your hormones. That’s how your baby gets whatever it wants.

Estrogen helps in the formation of blood vessels, giving you rosy skin, preps the breaks for milk production. Another hormone called Progesterone helps in supporting the developing embryo by maintaining the uterus’s inner layer.

The pregnancy nausea you experience in the first trimester is because of hCG (Human Chorionic Gonadotropin). hCG is a placental hormone.

Did you know?
The pregnancy test kits base their tests on the presence or absence of hCG in the urine to show a positive.

The growing human embryo cells initially produce hCG; these cells eventually form the placenta. hCG also stimulates the formation of estrogen and progesterone until the placental cells can do so by themselves.

As expected, there is a high demand for resources because you are now growing a baby from one tiny cell—your metabolic rate increases. The human placental lactogen increases this rate. It also helps in milk production.

So to-be-mamas, your baby has already got its way. Your baby is smart and is modifying its environment through the placenta.

Listen to your body. Take a nap when you feel tired. Drink enough water—snack with some fruits when you are hungry. Take a stroll if you feel like it!

Why does the mother’s immune system not reject the fetus?

Placenta : The shield against the mother's immune system
Placenta: The shield against the mother’s immune system

If a virus or a bacteria attacks us, our body reacts by releasing antibodies. These reactions help in expelling the infection or foreign bodies, which is the job of our immune system.

Now, a fetus gets half of its genetic material from the father. The mother’s immune system should technically perceive it as an intruder! Yet, the mother’s body shifts from rebel mode to ally mode in pregnancy. The mother nurtures, feeds, and makes peace with what it perceives as a foreign invader: the fetus. 

How does the maternal immune system tolerate the fetus and the placenta?

The immune cells in the womb are suppressed to prevent rejection.

In a way, you can consider pregnancy as an immunosuppressed state. This might ring a bell to you about why your OB-GYN told you to avoid eating raw food outside of the home or avoid overly crowded public spaces.

The fertilized egg produces a particular enzyme that suppresses the mom’s immune reaction, preventing her from fighting with the fetus’s cells. Once the placenta is formed, it helps maintain the truce between these two potential adversaries. 

Hence the placenta has a unique ability to create immunological tolerance.

How does the placenta provide immunity to babies?

Before birth, a baby doesn’t have a fully-developed immune system isn’t fully developed. It needs a helping hand from mom. It’s the mother who passes her immune-system warriors, called antibodies, through the placenta. This is called passive immunity because the fetus doesn’t make antibodies but accepts the mother’s antibodies.

A fetus’s immune system doesn’t start developing on its own until nine weeks and isn’t up and running until week fourteen. That delay is to help the fetus tolerate mom and not fight her from day one. 

The role of the placenta in neurological development

Researchers have discovered the role of the placenta that helps in the neurological development of the fetus.

A study published by British researchers showed that when a mother mouse is deprived of food, the placenta takes over, breaking down its tissue to nourish the fetal brain. Isn’t this fascinating?

Scientists at the University of Southern California’s Zilkha Neurogenetic Institute (ZNI) reported that the placenta—not the mother—provides the hormone serotonin to the fetus’s forebrain early in development. Because hormones play an essential role in brain wiring, even before they function as neurotransmitters in the brain, hence placental abnormalities could directly influence neurological development.

Does the placenta come out with the baby?

This life-sustaining organ develops quickly, nourishes the fetus, and orchestrates its death at the time of the baby’s birth. The mother delivers the placenta a few minutes after the delivery of your baby.

6 tips for a healthy placenta

Follow these tips for a healthy placenta. Mother with healthy placenta has a balanced outlook.

Several things have been shown to contribute to the optimal placental function. Follow the basics, and you should be on your way to a healthy pregnancy.

  1. Be active and regularly go on walks. Do some deep breathing exercises as they increase the supply of oxygen to your baby. However, don’t exercise excessively.
  2. Avoid exposure to high altitudes. Traveling to higher elevations exposes your body — pregnant or not — to lower air pressure and leads to lower oxygen levels in the blood. As a result, your tissues can become deprived of oxygen, a condition known as hypoxia
  3. Your blood pressure should be within normal limits. A mother’s blood pressure greatly influences how blood, nutrients, oxygen, and immune cells reach the fetus and affects the surface area available for efficient exchanges.
  4. Avoid stress. Stress, whether pregnant or not, weakens your immune system. When you are pregnant, you are already in an immunosuppressed state. So you do not want to add to the burden. You also run the risk of getting high BP with increased stress.
  5. Stay updated on your immunizations three to four months before you get pregnant. Your health provider will avoid any vaccines during pregnancy as you are in an immunosuppressed state.
  6. The best position to sleep: Lie on your sides. Of course, sleeping on your stomach is out of the question. But avoid lying flat on your back too. In this position, the weight of your uterus compresses the blood vessels that are feeding the placenta. Lying on your left side is better than lying on your right side because it allows more blood flow to the uterus.


Q: What is placenta?

A: Placenta is disc shaped tissue that connects the umbilical cord and uterus to deliver oxygen and nutrients to babies.

Q: When does the placenta form?

A: The placenta forms gradually in the first 3 months. After the fourth month, it grows at the same pace as the uterus.

Q: What does a placenta look like?

A: The placenta is bumpy tissue that looks like a dark red disc. The dark red color is because it is filled with blood vessels.

Q: How to tell where placenta is on ultrasound?

A: Usually, the placenta can be seen at the top of the uterus during an ultrasound in the 12th and 20th week.

Q: What is in placenta?

A: The placenta is mostly made up of blood vessels.

Q: How much does a placenta weigh?

A: According to the National Institutes of Health, in a full term pregnancy, the averagee weight of placenta is 508g.

More posts you might like:


  1. The placenta is key to the rise of mammals. Britannica. 2021. “Placental mammal.”
  2. The balance between the mom’s immune reaction and the baby’s development. ScienceDaily. 2012.Pregnancy: Why mother’s immune system does not reject developing fetus as foreign tissue.”
  3. Mother’s immune system and the fetus. Linköping University. 2015. “The placenta protects the fetus from the mother’s immune system.”
  4. Neurological development. PNAS. 2011. “Placental protection of the fetal brain during short-term food deprivation.”
  5. Role of the placenta. Nature. 2011. A transient placental source of serotonin for the fetal forebrain.”
  6. Avoid exposure to high altitudes during pregnancy. PubMed. 2012. “Travel to high altitude during pregnancy: frequently asked questions and recommendations for clinicians.
  7. Hypoxia during pregnancy. Insider. 2020. “Is it safe to travel to high altitudes when pregnant? Yes, to a point.
  8. Normal blood pressure during pregnancy. Medical News Today. 2020. “Normal blood pressure during pregnancy.
  9. Stress during pregnancy. Yale News. 2020. “The harmful effects of stress during pregnancy can last a lifetime.
  10. Vaccinations before pregnancy. CDC. 2021. “Vaccines Before Pregnancy.
  11. Best sleeping position during pregnancy.Healthline. 2021. “What Are the Best Sleeping Positions When You’re Pregnant?

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