After every delivery, the placenta gets a hands-on physical check right in the birthing room. In most cases, that bedside exam is all that happens. But when complications arise during pregnancy or delivery, the placenta is sent to a pathology lab for a much deeper examination that can reveal infections, blood flow problems, and clues about the baby’s health going forward.
The Bedside Exam Every Placenta Gets
Within minutes of delivery, your midwife or doctor examines the placenta by hand and eye. They’re looking at four things: whether the entire placental disc is intact with no missing pieces, whether the membranes (the “bag of waters”) are complete, the number of blood vessels in the umbilical cord, and any visible abnormalities like unusual color, odor, or texture. All of these findings are documented in your medical record.
Completeness is the most urgent concern. If a piece of the placenta or membranes is missing, tissue left behind in the uterus can cause heavy bleeding or infection. The cord vessels matter too. A normal umbilical cord has three vessels: two arteries and one vein. A single umbilical artery, found in about 1% of singleton pregnancies and up to 5% of twin pregnancies, is the most common cord abnormality and is linked to a higher risk of congenital anomalies in the cardiovascular, gastrointestinal, and genitourinary systems. Spotting this at delivery can prompt follow-up testing for the baby.
When the Placenta Goes to the Lab
Not every placenta needs laboratory analysis. A formal pathology exam is ordered when specific red flags were present during the pregnancy or delivery. Common reasons include:
- Pregnancy loss or stillbirth
- Preterm birth
- Severe preeclampsia
- Suspected placental abruption (the placenta pulling away from the uterine wall)
- Fetal growth restriction (a baby measuring significantly small)
- Signs of maternal infection during labor
- Emergency delivery due to abnormal fetal heart tracings
- A newborn showing signs of organ failure or neurological problems
- Multiple pregnancies (twins or higher)
- Anything visibly abnormal on the bedside exam
If none of these apply and both you and the baby are healthy, the placenta is typically discarded as medical waste. Guidelines for when to submit a placenta vary by country, but the United States, United Kingdom, and Australia all have formal criteria developed by teams of pathologists and obstetricians.
Testing for Infection and Inflammation
One of the most important things pathologists look for is inflammation caused by infection. Under the microscope, they check for immune cells (specifically neutrophils) infiltrating the placental tissues. Where those cells show up tells the story of what happened.
When immune cells invade the membranes surrounding the baby, this is called chorioamnionitis, a sign of ascending infection that most often starts as bacteria traveling up from the birth canal. When inflammation reaches the umbilical cord itself, affecting the vein first, then the arteries, and finally the surrounding jelly-like tissue, that indicates the baby’s own immune system was activated. Pathologists grade these findings by stage (how far the inflammation spread) and severity (how dense the immune cell clusters are), which helps doctors understand how significant the infection was.
These findings directly affect your baby’s care. Evidence of active infection in the placenta can prompt a sepsis workup for the newborn, and in some cases, doctors may start antibiotics even before blood culture results come back. If a baby is being evaluated for brain injury, signs of infection in the placenta will push the medical team to test for meningitis as well.
Blood Flow and Vascular Problems
The placenta is essentially a vascular organ, and problems with blood supply leave visible damage. Pathologists look for a pattern called maternal vascular malperfusion, which is the single most common abnormal finding in placentas from pregnancies complicated by preeclampsia, fetal growth restriction, and preventable stillbirths.
The diagnostic features include a placenta that weighs less than expected for gestational age, areas of infarction (tissue death from blocked blood flow), hemorrhage behind the placenta, and microscopic changes showing the tiny villi that exchange nutrients weren’t developing properly. Researchers have identified two main patterns: one combining a small placenta with tissue death from poor oxygen supply, strongly associated with preeclampsia plus a small baby, and another showing tissue death without the reduced size, linked to preeclampsia with a normal-weight baby. Both patterns, along with abnormalities in the blood vessels of the uterine lining, are particularly associated with deliveries before 34 weeks.
On the fetal side of circulation, pathologists also check for clotting problems in the baby’s placental blood vessels. This finding, called fetal thrombotic vasculopathy, can suggest the baby had clotting events before birth and may prompt additional neurological evaluation and monitoring.
Conditions That Affect Future Pregnancies
Some placental findings carry a high risk of recurring in your next pregnancy, which makes identifying them especially valuable. Three conditions stand out.
The most common is villitis of unknown etiology, an inflammation of the placental villi that isn’t caused by any identifiable infection. Despite its name suggesting mystery, it’s a well-recognized pattern that pathologists can diagnose on a microscope slide. Less common but more severe are chronic histiocytic intervillositis, where a specific type of immune cell floods the spaces between villi, and massive perivillous fibrin deposition, where clot-like material encases large portions of the villous tissue and chokes off nutrient exchange. All three conditions have recurrence rates ranging from 34% to 100% in subsequent pregnancies. Knowing this allows your care team to plan closer monitoring and, in some cases, preventive treatment the next time around.
Genetic Testing of Placental Tissue
In certain situations, placental tissue is sent for chromosomal or genetic analysis rather than (or in addition to) standard pathology. This is most relevant after a pregnancy loss, when testing the baby directly may not be possible.
Genetic testing of the placenta comes with an important caveat: the placenta and the baby don’t always share identical genetics. A condition called confined placental mosaicism means the placenta can carry chromosomal abnormalities that the baby does not, or vice versa. This is also why non-invasive prenatal testing (NIPT) done during pregnancy, which analyzes DNA fragments shed by the placenta into the mother’s blood, occasionally produces false positives. It’s reading the placenta’s DNA, not the baby’s directly. When mosaicism is suspected, additional testing on the baby is needed to confirm whether an abnormality is truly present.
How Results Shape Your Baby’s Care
Placental pathology results typically take days to weeks to come back, but they can meaningfully change how your baby is monitored and treated. Massive hemorrhage within the placenta or cord abnormalities like excessive coiling or true knots may prompt testing for bleeding disorders or organ damage that wasn’t obvious at birth. For babies admitted to the NICU with signs of brain injury, the placental report helps the medical team narrow down whether the cause was infection, a clotting event, or chronic oxygen deprivation, each of which calls for a different approach to treatment and follow-up.
For you, the results can explain what went wrong in a complicated pregnancy and, just as importantly, inform planning for future pregnancies. If your placenta showed vascular malperfusion linked to preeclampsia, or one of the high-recurrence inflammatory conditions, that information becomes part of your medical history and shapes the level of surveillance you’ll receive next time.