Placenta: Function, Anatomy, Pregnancy and Complications
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer: What is the placenta?
The placenta is a temporary pregnancy organ attached to the uterus. Through the umbilical cord, it supports the fetus with oxygen and nutrients, removes waste, produces pregnancy-related hormones and transfers antibodies. It forms during pregnancy, attaches to the uterine wall and connects to the fetus through the umbilical cord. [1]
Core idea | Plain-language meaning |
What it is | A temporary organ formed during pregnancy. |
Where it attaches | To the uterine wall. |
How it connects to the fetus | Through the umbilical cord. |
Main jobs | Oxygen and nutrient delivery, waste removal, hormone production and antibody transfer. |
Blood exchange | Materials are exchanged without the pregnant person's and fetus's blood mixing. |
After birth | It is delivered as the afterbirth, the third stage of labor. |
What the research says
Retained placenta is a leading cause of postpartum bleeding, and placenta previa is a strong risk factor. A systematic review and meta-analysis of 327 studies including 847,413,451 women found that retained placenta accounted for 16.4% (95% CI 12.3–20.5%) of postpartum haemorrhage causes and abnormal placentation for 3.9% (95% CI 0.1–7.6%). Placenta praevia was among the risk factors with a strong association with postpartum haemorrhage (Yunas et al., Lancet, 2025). [2]
Placenta accreta becomes more common with more prior cesarean deliveries. A systematic review and meta-analysis of 14 cohort studies covering 3,889 pregnancies with placenta previa or a low-lying placenta and at least one prior cesarean found placenta previa accreta in 328 pregnancies (8.4%). The incidence was 4.1% with one prior cesarean and 13.3% with two or more, and ultrasound diagnosed 90.9% of cases before birth (Jauniaux & Bhide, Am J Obstet Gynecol, 2017). [3]
Several factors are linked to placenta previa. An umbrella review of nine meta-analyses found highly suggestive evidence linking placenta previa to advanced maternal age (OR 3.16, 95% CI 2.79–3.57), prior cesarean (OR 1.60, 95% CI 1.44–1.76) and smoking (OR 1.42, 95% CI 1.30–1.54) (Jenabi et al., Placenta, 2022). [4]
What is the placenta?
The placenta is an organ that develops in the uterus during pregnancy. It is temporary: it forms to support the fetus and is delivered after the baby is born. [1]
The placenta attaches to the uterine wall, while the umbilical cord connects the fetus to the placenta. Together, they allow nutrients from the pregnant person's body to support fetal growth. [1]
What does the placenta do?
The placenta is vital to fetal development. Until birth, it acts as the fetus's lungs, kidneys and liver. [1]
Its main functions are to provide oxygen and nutrients, remove harmful waste products and carbon dioxide, produce hormones that maintain pregnancy and support fetal growth, and pass immunity from the pregnant person to the fetus. [1]
Placental function | What it means |
Oxygen delivery | Oxygen reaches the fetus through the placenta and umbilical cord. |
Nutrient delivery | Glucose and other nutrients support fetal growth. |
Waste removal | Harmful waste and carbon dioxide move away from the fetus's blood. |
Hormone production | Placental hormones help maintain pregnancy and support growth. |
Antibody transfer | Antibodies pass to the fetus near delivery and help begin immunity. |
How does the placenta exchange nutrients and waste?
Blood passes through the placenta and provides oxygen, glucose and nutrients to the fetus through the umbilical cord. The placenta can also filter harmful waste and carbon dioxide out of the fetus's blood. [1]
The two bloodstreams exchange these materials without the blood supplies mixing. Near delivery, antibodies from the pregnant person's immune system pass through the placenta to the fetus. This immunity stays with the baby for several months after birth. [1]
Which hormones does the placenta make?
The placenta acts as an endocrine organ. It makes several hormones that affect both the pregnant person and the fetus. [1]
Hormone | What it does |
Estrogen | Helps the uterus grow, eventually allows it to contract and supports mammary-gland development for lactation. |
Progesterone | Maintains the uterine lining so the placenta and fetus can grow. |
Human chorionic gonadotropin (hCG) | The first hormone the placenta releases. Pregnancy tests measure it, and it signals the corpus luteum to maintain progesterone production. |
Human placental lactogen (hPL) | May support mammary-gland growth and help regulate metabolism during pregnancy. |
Placental growth hormone (PGH) | Helps control fetal growth. |
Relaxin | Relaxes ligaments and muscles and softens the cervix near the end of pregnancy. |
Progesterone and human placental lactogen stimulate appetite, which is one reason many people feel extra hungry during pregnancy. [1]
When does the placenta form?
The placenta begins forming after a fertilized egg implants in the uterine lining, also called the endometrium. It has the same genetic makeup as the fetus. [1]
It continues to grow throughout pregnancy. It starts as a few cells and grows to several inches long while supporting the fetus. [1]
When does the placenta take over hormone production?
By the end of the first trimester, around 12 weeks of pregnancy, the placenta takes over most hormone production. Until then, the corpus luteum handles most of it. [1]
Many people's first-trimester nausea and fatigue ease in the second trimester after this transition. This is not universal, so individual pregnancy experiences can differ. [1]
Where is the placenta, and does it move?
The placenta can form anywhere on the inner lining of the uterus, wherever the fertilized egg implants. A low-lying placenta sits close to the cervix; when it covers part or all of the cervix, it is called placenta previa. [1]
The placenta appears to move only because the uterus expands as pregnancy continues. Most placentas "move" toward the top or side of the uterus by about 32 weeks. A pregnancy-care professional checks placental position during the 20-week anatomy ultrasound to see whether it may cause complications. [1]
How is the placenta delivered?
The placenta is delivered shortly after the baby is born, usually within 30 minutes. This is called the afterbirth and is the third stage of labor. [1]
After a vaginal delivery, continued uterine contractions help expel the placenta. A healthcare professional may press on the abdomen or ask for one final push. During a C-section, the placenta is removed through the same incision used for the baby's delivery. [1]
Birth setting | How the placenta is delivered |
Vaginal delivery | Uterine contractions help remove it; a professional may press on the abdomen or ask for a final push. |
C-section | A professional removes it through the same incision used for delivery. |
What happens to the placenta after birth?
After delivery, a healthcare professional checks the placenta to confirm that it detached completely from the uterus and to look for missing pieces or breakage. It is generally discarded unless the family chooses otherwise. [1]
Some people choose placental tissue banking, which collects and stores placental tissue and blood after delivery. Placental tissue is rich in stem cells that can help treat certain conditions. [1]
A pathologist may examine the placenta if the pregnant person or baby has a fever or illness, if the baby is born prematurely or if the baby is small for gestational age. [1]
What are common placental complications?
A placental condition can affect the pregnant person, the fetus or both. Common examples are placenta previa, placenta accreta, placental abruption, placental insufficiency and retained placenta. [1]
Condition | Description |
Placenta previa | The placenta covers all or part of the cervix. |
Placenta accreta | The placenta attaches too deeply to the wall of the uterus. |
Placental abruption | The placenta separates from the uterus too early. |
Placental insufficiency | The placenta isn't providing enough nutrients or oxygen to the fetus. |
Retained placenta | Part of the placenta stays inside the uterus after delivery and can cause excessive bleeding and/or infection. |
The placenta may also be involved in other pregnancy conditions. Placental hormones can cause insulin resistance, which can lead to gestational diabetes, and some researchers think that preeclampsia may relate to a problem with the placenta's blood supply. A fetus measuring small for its gestational age, known as intrauterine growth restriction, can also be a sign of a placental problem. [1]
These associations do not provide an individual risk assessment. Each condition has different risk factors and symptoms, which is why attending prenatal appointments matters.
What symptoms may need medical attention?
Bleeding from the vagina is the most common sign of a placental problem, although it has other causes too. Abdominal pain, contractions and decreased fetal movement are other possible signs. [1]
After delivery, excessive bleeding or signs of infection can occur when placental tissue remains in the uterus. These symptoms can have multiple causes. They do not establish a placental diagnosis, and an online article cannot determine how urgent an individual situation is.
Situation | Why to contact a pregnancy-care professional |
Vaginal bleeding | The most common sign of a placental problem, though it has other causes too. |
Abdominal pain | A possible sign of a placental problem. |
Contractions | A possible sign of a placental problem. |
Reduced fetal movement | A concerning pregnancy sign that should be discussed promptly. |
Fever or illness | May lead a professional to ask a pathologist to examine the placenta around delivery. |
Excessive bleeding after delivery | Can occur when placental tissue remains in the uterus. |
Can you do anything to improve placental health?
Placental health is not something a person can do much to change, but prenatal appointments and monitoring remain important. Prenatal appointments and professional monitoring remain important parts of pregnancy care.
Rather than trying to diagnose or change the placenta independently, discuss symptoms, test results and pregnancy concerns with the clinician overseeing your care.
How is the placenta checked during pregnancy?
A pregnancy-care professional looks at the placenta's location during the 20-week anatomy ultrasound and checks whether its position, such as a low-lying placenta, may cause complications. Which examinations or imaging studies are appropriate depends on the individual pregnancy. [1]
A placenta's location or appearance on imaging should be interpreted in the context of the full pregnancy and the report from the care team.
When should you contact a pregnancy-care professional?
Contact a qualified pregnancy-care professional for vaginal bleeding, abdominal pain, contractions, reduced fetal movement, fever, illness or other concerning pregnancy symptoms. After delivery, seek professional assessment for excessive bleeding or possible infection.
Call 911 or go to the hospital right away for heavy vaginal bleeding during pregnancy, severe belly pain, a baby who is moving much less than usual, or after birth bleeding that soaks a pad in an hour or less or comes with dizziness or fainting. Do not rely on this article to decide whether a symptom is normal for you or your pregnancy.
Conclusion: A temporary organ with essential work
The placenta is a temporary organ that attaches to the uterus and connects with the fetus through the umbilical cord. It supplies oxygen and nutrients, removes waste and carbon dioxide, produces hormones and transfers antibodies. [1]
It forms after implantation, grows throughout pregnancy and takes over most hormone production by around 12 weeks. Shortly after birth, it is delivered as the afterbirth. Placental conditions can cause complications, and symptoms such as bleeding, abdominal pain, contractions, reduced fetal movement or excessive postpartum bleeding deserve professional attention. [1]
Use this guide to prepare questions for your pregnancy-care team. Clear information is a starting point for better care, not a diagnosis. [1]
Frequently Asked Questions
What is the placenta?
The placenta is a temporary organ that forms in the uterus during pregnancy, attaches to the uterine wall and connects with the fetus through the umbilical cord.
What does the placenta do?
The placenta provides oxygen and nutrients, removes fetal waste and carbon dioxide, produces hormones that support pregnancy and fetal growth, and passes antibodies to the fetus.
Does the pregnant person's blood mix with the baby's blood?
No. The placenta allows exchange between the two blood supplies without the blood mixing.
Which hormones does the placenta make?
Named hormones include estrogen, progesterone, human chorionic gonadotropin (hCG), human placental lactogen, placental growth hormone and relaxin.
When does the placenta take over hormone production?
By the end of the first trimester, around 12 weeks of pregnancy, the placenta takes over most hormone production from the corpus luteum.
When is the placenta delivered?
Usually within 30 minutes after the baby, during the third stage of labor, also called the afterbirth.
What happens if part of the placenta remains after delivery?
Retained placental tissue can cause excessive bleeding and/or infection and needs professional assessment.
What are placenta previa, placenta accreta and placental abruption?
Placenta previa is when the placenta covers all or part of the cervix. Placenta accreta is when it attaches too deeply to the uterine wall. Placental abruption is when it separates from the uterus too early.
What symptoms should I discuss with a pregnancy-care professional?
Vaginal bleeding, abdominal pain, contractions, reduced fetal movement or other concerning symptoms. After delivery, seek assessment for excessive bleeding or signs of infection.
Can I improve placental health?
There is not much a person can do to affect placental growth and health. Prenatal care and professional monitoring remain important.
Related guides
References
Cleveland Clinic. Placenta: Purpose, Anatomy, Function & Complications. Cleveland Clinic. Updated December 15, 2025. https://my.clevelandclinic.org/health/body/22337-placenta
Yunas I, Islam MA, Sindhu KN, et al. Causes of and risk factors for postpartum haemorrhage: a systematic review and meta-analysis. Lancet. 2025;405(10488):1468-1480. PMID 40188841. https://doi.org/10.1016/S0140-6736(25)00448-9
Jauniaux E, Bhide A. Prenatal ultrasound diagnosis and outcome of placenta previa accreta after cesarean delivery: a systematic review and meta-analysis. Am J Obstet Gynecol. 2017;217(1):27-36. PMID 28268196. https://doi.org/10.1016/j.ajog.2017.02.050
Jenabi E, Salimi Z, Bashirian S, et al. The risk factors associated with placenta previa: An umbrella review. Placenta. 2022;117:21-27. PMID 34768164. https://doi.org/10.1016/j.placenta.2021.10.009
Disclaimer
This article is for general educational purposes and is not a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified healthcare professional with any questions about a medical condition or treatment.
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