The Placenta: When It Forms, When It Takes Over and What It Does

Ask a room of new parents what the placenta looked like and you get a pause, then a shrug. Most people spend nine months relying on an organ they never think about, then hand it to a midwife or nurse and forget it existed. It is the only organ the body builds from scratch, uses once and discards on purpose.

The questions that do come up tend to arrive around week nine or ten, usually from someone lying on a bathroom floor: when does this get easier, and is it true the placenta “takes over”? The short version is yes, there is a real handover in early pregnancy, and it explains a surprising amount about how the first trimester feels.

What follows is the placenta’s biography: how it forms, the moment it assumes hormonal control, what it does for the next six months and what actually happens when it is done.

What is the placenta, in plain terms?

Picture a flat, disc-shaped organ pressed against the inside wall of the uterus, dark red on the side that faces the mother and smooth and gray-blue on the side that faces the baby. A cord runs from its center to the baby’s abdomen. That is the placenta, and it belongs to both of them: it is grown mostly from the embryo’s own cells, yet it is embedded in and fed by the mother’s tissue.

Its job is exchange. The mother’s blood pools in spaces within the placenta, and the baby’s blood flows through tiny finger-like vessels called villi that dip into those pools. The two bloodstreams never mix, but they come within a few cells’ thickness of each other. Oxygen, glucose, amino acids and antibodies pass one way; carbon dioxide and waste pass the other.

The placenta is also a hormone factory. According to Cleveland Clinic, it produces human chorionic gonadotropin (hCG), progesterone, estrogen and human placental lactogen, among others, and by the end of the first trimester it is responsible for most of the hormone output that keeps a pregnancy going.

Think of it as lungs, kidneys, gut and endocrine gland for a passenger who cannot yet use their own. When people describe it as a life-support system, that is not a metaphor.

When does the placenta start to form?

Earlier than most people assume. The placenta begins as a layer of cells on the outside of the fertilized egg, called the trophoblast. When the egg implants in the uterine lining, roughly six to ten days after fertilization, those outer cells burrow into the wall and start recruiting a blood supply. In pregnancy dating terms, which count from the first day of the last period, that is around week three to four, often before a home test would turn positive.

Through weeks four to eight the structure branches and thickens. The villi form, the maternal blood spaces open up and the umbilical cord develops. Mayo Clinic notes that the placenta attaches to the uterine wall in these early weeks and grows in step with the baby throughout pregnancy.

By about 12 weeks it is recognizable as a mature organ. The NHS week-by-week guide describes the placenta as fully formed by this point, though it continues to grow in size and blood flow for months.

Two things worth knowing from this timeline:

• Early ultrasound scans usually show a developing placenta before they show a clearly formed baby, because the placental tissue is doing the heavy lifting first.
• The hormone hCG that makes a pregnancy test positive is made by these early placental cells, not by the embryo itself.

So the organ that will sustain a pregnancy is quietly at work before the pregnancy has been confirmed.

When does the placenta take over from the corpus luteum?

Here is the handover people are asking about. After ovulation, the empty follicle in the ovary becomes a small hormone gland called the corpus luteum. Its job is to make progesterone, which keeps the uterine lining thick and stable. In a non-pregnant cycle it fades after about two weeks and a period follows.

Once an embryo implants, the young placenta produces hCG, and hCG’s main early purpose is to tell the corpus luteum not to shut down. The corpus luteum keeps making progesterone for the first several weeks while the placenta builds capacity.

Gradually, the placenta’s own progesterone output rises until it no longer needs the ovary. Cleveland Clinic places this transition at the end of the first trimester, around 12 weeks. Clinicians often describe the shift as beginning around week 8 and being complete by week 10 to 12; it is a ramp, not a switch.

Once the placenta has taken over, hCG is no longer needed in such quantities. MedlinePlus reference ranges show hCG climbing steeply through the early weeks, peaking somewhere between about weeks 8 and 12, then falling to a lower plateau for the rest of pregnancy.

The takeover matters clinically because it marks the point where the pregnancy is sustained by an organ built for the job rather than by a temporary gland. That is one reason the first-trimester scan and the 12-week milestone carry so much weight.

Why do first-trimester symptoms often ease around this time?

Nausea, exhaustion and tender breasts track closely with hCG. When hCG is climbing fast, those symptoms tend to peak; when it plateaus and falls after the placenta assumes control, many people notice relief. The overlap is suggestive rather than proven, since nobody has isolated one hormone as the sole cause, but the timing lines up well.

The NHS notes that nausea and vomiting in pregnancy usually clear up by weeks 16 to 20, and for many people improvement starts earlier, around the end of the first trimester. That window sits right after the hormonal handover.

What the evidence does not support is the idea that the placenta taking over should make you feel better on a specific day. Some people feel dramatically improved at 11 weeks; others carry nausea well into the second trimester with a perfectly healthy placenta. A minority experience severe, persistent vomiting that needs medical care regardless of week.

Fatigue follows a similar arc. Progesterone has a sedating effect and early pregnancy demands a large increase in blood volume and cardiac output. As the placenta matures and the body adapts, energy often returns in the second trimester.

The honest summary: the handover is real, symptoms frequently ease around it, and the two are probably related. Feeling fine at 9 weeks or rough at 15 weeks does not, by itself, tell you anything about how the placenta is doing.

What does the placenta actually do all day?

Four jobs, running simultaneously, without pause, for roughly 40 weeks.

The first is breathing. The baby’s lungs are filled with fluid and do not exchange gas until birth. Oxygen dissolved in the mother’s blood diffuses across the placental membrane into the baby’s circulation, and carbon dioxide travels back. Mayo Clinic describes this as the placenta providing oxygen and nutrients to the baby and removing waste products from the baby’s blood.

The second is feeding. Glucose, amino acids, fatty acids, vitamins and minerals cross by a mix of diffusion and active transport, where placental cells spend energy pumping specific molecules across. Iron, for instance, is actively moved toward the baby even when the mother’s own stores are modest.

The third is waste disposal. Urea and other byproducts pass back to the mother, whose kidneys and liver deal with them.

The fourth is defense. In the third trimester the placenta transfers maternal antibodies, particularly the IgG class, giving the newborn passive immunity that lasts for months after birth. This is the mechanism behind the observation that babies are relatively protected from certain infections in early life.

Layered over all four is hormone production, covered in the next section. No other organ combines these roles, and none is expected to build itself while doing them.

Which hormones does the placenta make, and what do they do?

The placenta’s hormone output changes the mother’s physiology from head to toe. The table below sets out the main players described by Cleveland Clinic and Mayo Clinic.

Hormone	Main role in pregnancy	Notable pattern
hCG	Sustains the corpus luteum in early weeks; basis of pregnancy tests	Rises steeply, peaks around weeks 8 to 12, then declines
Progesterone	Keeps the uterine lining stable, relaxes uterine muscle, prevents contractions	Made by the ovary first, then by the placenta from about 10 to 12 weeks
Estrogen	Drives uterine and breast growth, increases blood flow to the uterus	Rises steadily to term
Human placental lactogen	Adjusts maternal metabolism so more glucose is available to the baby; prepares breast tissue	Increases with placental size
Relaxin	Loosens ligaments and softens the cervix	Present throughout, contributing to joint laxity

Human placental lactogen deserves a moment. Its effect on glucose handling is part of why insulin resistance rises in the second half of pregnancy, which is normal and purposeful. In some people that shift tips into gestational diabetes, which is why screening happens in the second trimester rather than the first.

Progesterone’s muscle-relaxing effect is not confined to the uterus. It slows the gut, loosens the valve at the top of the stomach and relaxes blood vessel walls, which is a plausible mechanism for pregnancy constipation, heartburn and dizziness on standing.

How does the placenta move things between you and the baby?

The membrane separating the two circulations is astonishingly thin, a few cells thick at term, and it is selective rather than sealed. Understanding that distinction clears up a stubborn misconception.

Small molecules such as oxygen, carbon dioxide and water cross by simple diffusion, moving from higher concentration to lower. Glucose uses carrier proteins. Amino acids and some minerals are actively pumped. Large proteins generally cannot cross, with the important exception of IgG antibodies, which are carried across by a dedicated receptor.

Here is where the myth lives. Many people believe the placenta acts as a filter that keeps harmful substances away from the baby. It does not, at least not reliably. Alcohol crosses freely. Nicotine and carbon monoxide from smoking cross and reduce oxygen delivery. Caffeine crosses. Many medications cross, which is exactly why prescribers weigh each one individually in pregnancy and why any decision about starting or stopping a medicine belongs with the prescribing clinician rather than a general article.

Some infections cross too. Certain viruses and the parasite responsible for toxoplasmosis can reach the baby through the placenta, which is the reason for food-safety advice and infection screening in antenatal care.

What the placenta does do well is metabolize some substances and limit others, so it is a partial barrier. The practical takeaway is that the phrase “the placenta will protect the baby” overstates the case, and the standard advice to avoid alcohol and smoking in pregnancy rests on how readily those substances pass through.

How big does the placenta get, and how does it grow?

At the end of the first trimester the placenta is often larger than the baby it serves. That imbalance flips in the second trimester, when the baby’s growth accelerates and the placenta’s slows, but the organ never stops enlarging.

By birth, Cleveland Clinic describes a typical placenta as weighing about one pound, roughly 22 centimeters across and 2 to 3 centimeters thick, about the size and shape of a dinner plate. A common rule of thumb is that a healthy placenta weighs about one-sixth of the baby’s birth weight, though that ratio varies.

Growth is not just size. Blood flow through the uterine arteries increases enormously across pregnancy, and the villi branch more finely so that the exchange surface expands. By term the total surface area of the placental membrane, if it could be laid flat, would cover several square meters.

Ultrasound cannot measure function directly, but it can assess placental position, appearance and, when there is concern, blood flow in the umbilical cord using Doppler. A placenta that is unusually small or thick, or a baby growing more slowly than expected, may prompt closer monitoring. The Mayo Clinic notes that factors such as high blood pressure, smoking and being older at the time of pregnancy can affect placental health, which is one reason antenatal appointments track blood pressure and growth so consistently.

None of these measurements are things to worry about at home. They are the reason routine scans exist.

Does it matter where the placenta is attached?

Scan reports often mention the placenta’s position, and the terms can sound more alarming than they are. Anterior means it sits on the front wall of the uterus, toward the abdomen. Posterior means the back wall, toward the spine. Fundal means the top. Lateral means to one side. All of these are normal.

An anterior placenta has one practical consequence: it cushions the front of the uterus, so kicks may be felt later and more faintly, particularly in a first pregnancy. The NHS says most people first notice movements between 16 and 24 weeks, and an anterior placenta can push that toward the later end. Once a pattern is established, though, the advice is the same for everyone: get to know what is normal for your baby and report any reduction.

The position that does require follow-up is low-lying. If the placenta reaches or covers the cervix, it is called placenta praevia. The NHS reports it affects about 1 in every 200 births. Many placentas that look low at the 20-week scan have moved clear of the cervix by the third trimester, not because the placenta migrates but because the lower part of the uterus stretches upward and outward. A repeat scan around 32 weeks is standard to check.

Position is not something anyone can influence. It is determined at implantation, weeks before most people know they are pregnant, and in the great majority of cases it makes no difference to the pregnancy at all.

What can go wrong with the placenta?

Most placentas do their job without incident. When problems occur, they fall into a handful of patterns that antenatal care is designed to catch. The NHS and Mayo Clinic describe the main ones.

Placenta praevia, mentioned above, is a placenta lying over or near the cervix. It can cause painless bleeding in later pregnancy and usually means a planned cesarean birth, because the placenta blocks the baby’s exit.

Placental abruption is when the placenta separates from the uterine wall before birth. It can cause bleeding, constant abdominal or back pain and a hard, tender uterus, and it is a genuine emergency because separation cuts oxygen supply. High blood pressure, smoking and abdominal injury raise the risk.

Placenta accreta is when the placenta grows too deeply into the uterine wall and does not detach cleanly after birth. It is more common after previous cesarean births or uterine surgery and is usually identified on scans in advance, allowing a planned delivery with the right team.

Placental insufficiency is a functional problem rather than a structural one: the placenta is not delivering enough oxygen and nutrients, and the baby grows more slowly than expected. It is associated with high blood pressure, pre-eclampsia, smoking and some medical conditions, and it is monitored through growth scans and Doppler measurements.

Retained placenta, where part or all of the placenta stays in the uterus after birth, is covered below.

What links these conditions is that they are detected through observation, whether that is a scan, a blood pressure reading or a person reporting bleeding. That is the argument for attending every appointment and speaking up early.

What happens to the placenta at birth?

Labor has three stages, and the placenta owns the third. After the baby is born, the uterus keeps contracting, shrinking around the placenta until it shears away from the wall. It is then pushed out through the vagina, cord attached, along with the membranes that surrounded the baby.

The NHS describes two approaches. With active management, a medication is given to help the uterus contract and the cord is gently guided; the stage typically takes 5 to 15 minutes. With physiological management, the body is left to complete the process on its own, which can take up to an hour. The choice is discussed in advance and can change during labor depending on circumstances. Which approach is used, and what is given, is a decision for the birth team.

Once delivered, the placenta is examined to check it is complete. If a fragment remains, the uterus may not contract fully, which raises the risk of heavy bleeding and infection. Retained placenta is one of the recognized causes of postpartum hemorrhage, and it is treated by removing the remaining tissue.

Cord clamping timing is also part of this stage. Waiting at least a minute before clamping lets more blood flow from the placenta to the baby, and delayed clamping is now standard practice in many guidelines for healthy babies.

After that, the placenta’s work is finished. Roughly nine months of continuous service, then a life span measured in minutes on the other side of birth.

Placenta myths worth retiring

Because the placenta is rarely seen and poorly understood, it attracts folklore. Three claims come up often enough to address directly.

The first is that eating the placenta after birth improves mood, milk supply or energy. The practice, sometimes called placentophagy, usually involves having the organ dried and encapsulated. There is no good-quality human evidence that it delivers those benefits, and there is a documented safety concern: the CDC reported a case in which a newborn developed a late-onset group B streptococcus infection and the same bacteria were found in the mother’s placenta capsules, with the report noting the preparation process may not kill all pathogens. People considering it deserve that information, without judgment.

The second is that the placenta filters out everything harmful. As covered earlier, it is selective, not protective. Alcohol, nicotine and many medications cross.

The third is that the placenta “moves” during pregnancy. A low placenta at 20 weeks that is clear of the cervix at 32 weeks has not traveled; the uterus has grown beneath it. The distinction matters because it explains why a repeat scan is reassuring rather than mysterious.

A smaller myth: that the side of the uterus the placenta attaches to predicts the baby’s sex. It does not. Position is random and has no relationship to chromosomes.

Retiring these ideas is not about being contrarian. The real story of the placenta is more interesting than any of them.

When to see a doctor about placenta-related symptoms

Most placental problems announce themselves through a short list of symptoms, and every one of them warrants a same-day call to a midwife, obstetric unit or emergency service rather than waiting for the next appointment.

Seek urgent care for any of the following during pregnancy:

• Vaginal bleeding of any amount, especially after 20 weeks, whether or not it is painful.
• Constant abdominal or lower back pain, or a uterus that feels hard and tender and does not soften.
• A noticeable reduction in the baby’s movements, or a change from the usual pattern, once movements are established.
• Regular contractions before 37 weeks.
• Sudden swelling of the face or hands, severe headache, visual disturbance or pain under the ribs on the right, which can signal pre-eclampsia, a condition closely tied to placental function.
• Fluid leaking from the vagina, or a fall or blow to the abdomen.

After birth, contact care urgently for heavy bleeding that soaks a pad in an hour or less, large clots, fever, foul-smelling discharge or worsening abdominal pain, all of which can indicate retained tissue or infection.

It is also entirely reasonable to call with a question that turns out to be nothing. Maternity services expect these calls; they are built around them. The NHS is explicit that reduced movements should never be left until the following day, and that advice applies equally to bleeding and persistent pain. Trust the instinct that something has changed and let a clinician decide whether it matters.

What matters most about the placenta, in one editor's view

If a single idea should survive from this article, it is that the placenta is an organ with a timeline, and knowing that timeline makes pregnancy less mysterious.

It forms in the first days after implantation, well before a positive test. It takes over hormonal control by about 10 to 12 weeks, which is why the end of the first trimester feels like a turning point for so many people and why clinicians treat it as one. It then works continuously until the third stage of labor, exchanging gases, nutrients and antibodies through a membrane that is selective but not impermeable.

Two practical conclusions follow. First, the choices that affect the placenta most, avoiding smoking and alcohol, managing blood pressure and attending antenatal appointments, matter from the very beginning, because the organ is under construction from week three. Second, the warning signs of placental trouble are few and specific: bleeding, persistent pain, reduced movements. Knowing them is more useful than knowing anything about placental position or size.

The placenta does not need admiration, but it probably deserves a little. It builds itself, runs four organ systems at once for a person who cannot yet do so, then leaves quietly when the job is done. Understanding when it forms and when it takes over is, in the end, understanding how a pregnancy holds itself together.

Frequently asked questions

When does the placenta take over hormone production?

By about 10 to 12 weeks of pregnancy, the placenta has taken over progesterone production from the corpus luteum in the ovary. The shift is gradual, beginning around week 8 and completing near the end of the first trimester, according to Cleveland Clinic. After the handover, hCG levels fall from their peak because the corpus luteum no longer needs to be sustained.

At what week does the placenta start forming?

The placenta starts forming at implantation, roughly six to ten days after fertilization, which corresponds to about week three to four of pregnancy when dating from the last period. The outer cells of the embryo burrow into the uterine lining and begin building blood vessels and villi. By around 12 weeks the placenta is fully formed, though it keeps growing until birth.

Is the placenta fully formed at 12 weeks?

Yes, the placenta is generally described as fully formed by about 12 weeks, when all its main structures are in place and it has taken over hormone production. Fully formed does not mean fully grown: it continues to increase in size, weight and blood flow throughout the second and third trimesters, reaching roughly one pound at term.

Does morning sickness stop when the placenta takes over?

For many people nausea eases after the placenta takes over, but not on a predictable day. The NHS notes that pregnancy sickness usually clears by weeks 16 to 20, and improvement often begins around the end of the first trimester as hCG falls. Some people feel better at 10 weeks; others have symptoms well into the second trimester with a healthy pregnancy.

What does the placenta do for the baby?

The placenta delivers oxygen and nutrients from the mother’s blood to the baby, removes carbon dioxide and waste, transfers protective antibodies in the third trimester and produces the hormones that maintain pregnancy. It does all this without the two bloodstreams mixing, across a membrane only a few cells thick, functioning as lungs, kidneys, gut and endocrine gland until birth.

Does an anterior placenta cause problems?

An anterior placenta, attached to the front wall of the uterus, is a normal position and does not usually cause problems. Its main effect is cushioning, so movements may be felt later and more faintly, especially in a first pregnancy. Once a pattern of movements is established, any reduction should still be reported promptly, exactly as with any other placental position.

Can a low-lying placenta move up?

A low-lying placenta often appears to move up, but the placenta itself does not travel. As the uterus grows, the lower segment stretches upward and outward, carrying the placenta away from the cervix. A repeat scan around 32 weeks checks whether it has cleared. If the placenta still covers the cervix, a planned cesarean birth is usually recommended.

How is the placenta delivered after birth?

The placenta is delivered in the third stage of labor, after the baby. The uterus contracts, the placenta separates from the wall and is pushed out with the membranes. With active management, which includes a medication to help the uterus contract, this usually takes 5 to 15 minutes; with physiological management it can take up to an hour, according to the NHS.

Is eating the placenta safe or beneficial?

There is no good-quality human evidence that consuming the placenta improves mood, energy or milk supply. There is a documented safety concern: the CDC reported a newborn with a late-onset group B streptococcus infection whose mother had taken placenta capsules containing the same bacteria. Anyone considering the practice should discuss it with their maternity team.

What are warning signs of a placenta problem?

Vaginal bleeding, constant abdominal or back pain, a hard tender uterus, reduced fetal movements and contractions before 37 weeks are the main warning signs and all need same-day assessment. Sudden swelling, severe headache or visual changes can indicate pre-eclampsia, which is linked to placental function. After birth, heavy bleeding, fever or foul-smelling discharge also require urgent care.

References

• Cleveland Clinic — Placenta
• NHS — The stages of labour and birth
• MedlinePlus — HCG blood test – quantitative
• CDC MMWR — Late-Onset Infant Group B Streptococcus Infection Associated with Maternal Consumption of Capsules Containing Dehydrated Placenta

This article is for general information only and is not a substitute for professional medical advice. Please consult a qualified doctor about your individual situation.