The Role of Placental Pathology in Understanding COVID-19 in Pregnancy

Pregnancy changes the immune, vascular, and respiratory systems in ways that can alter the course of viral infection. During the COVID-19 pandemic, clinicians therefore needed to understand how SARS-CoV-2 affected the pregnant patient, the placenta, and the developing fetus as connected parts of one biological system.

Placental pathology offered an important way to study that system. Examination of the placenta after delivery could reveal maternal vascular malperfusion, fetal vascular injury, inflammation, thrombotic lesions, and evidence of tissue damage associated with severe maternal illness. These findings helped researchers investigate whether COVID-19 caused direct placental infection, indirect injury through maternal inflammation, or a combination of both.

The evidence has required careful interpretation. A lesion found in a placenta is not automatically caused by SARS-CoV-2, and the detection of viral material does not always prove productive infection or fetal transmission. Meaningful conclusions depend on clinical history, gestational age, laboratory testing, gross examination, histology, and neonatal follow-up.

Why The Placenta Matters In Maternal Infection

The placenta is a temporary organ with several essential functions. It transfers oxygen and nutrients, removes waste, regulates endocrine signals, and creates an immunological interface between mother and fetus. Its vascular network must remain functional throughout pregnancy, so systemic inflammation, endothelial dysfunction, fever, hypoxia, or coagulation abnormalities may leave recognizable pathological footprints.

COVID-19 raised particular concern because severe disease can involve endothelial injury, exaggerated inflammatory responses, platelet activation, and abnormal coagulation. Similar processes have been associated with placental lesions in other maternal disorders, including preeclampsia, chronic hypertension, diabetes, and autoimmune disease. Pathologists therefore had to distinguish a COVID-19-associated pattern from background conditions that were already capable of damaging placental circulation.

The placenta also records events over time. Some lesions indicate an acute process near delivery, while others reflect repeated or longstanding vascular stress. This time-sensitive record can help clinicians connect maternal symptoms, hospitalization, oxygen requirement, medication exposure, fetal growth, and delivery timing with tissue findings.

Pathways Of Placental Involvement

A central question was whether SARS-CoV-2 could cross the maternal-fetal interface and infect placental cells. The virus uses the ACE2 receptor and related cellular machinery, but receptor expression varies by cell type, gestational age, and tissue state. The presence of a possible entry pathway does not establish that infection will occur efficiently in every placenta.

Researchers have investigated viral RNA, viral proteins, and signs of replication in placental tissue. These methods answer different questions. Polymerase chain reaction may detect genetic material, immunohistochemistry may localize antigen, and in situ hybridization may show where viral RNA is situated. Electron microscopy can contribute visual information, but particle-like structures must be interpreted carefully because cellular organelles and debris may resemble viral particles.

Indirect injury may be more common than direct placental infection. Maternal hypoxemia, fever, systemic inflammation, endothelial dysfunction, and altered coagulation can reduce effective placental exchange without requiring substantial viral replication inside placental cells. This distinction matters because it affects how clinicians assess fetal risk and how researchers describe vertical transmission.

Reading Histologic Patterns With Care

Placental examination in pregnancies affected by COVID-19 has frequently focused on maternal vascular malperfusion. Lesions may include decidual arteriopathy, accelerated villous maturation, increased syncytial knots, distal villous hypoplasia, and placental infarction. These changes can indicate reduced maternal blood flow through the intervillous space, although they are not specific to coronavirus infection.

Fetal vascular malperfusion has also received attention. Thrombi in fetal vessels, avascular villi, and other obstructive lesions may suggest impaired circulation within the fetal placental vascular tree. Their clinical significance depends on the extent and distribution of the lesions, as well as on fetal growth, Doppler findings, stillbirth risk, and neonatal condition.

Inflammatory patterns require similar caution. Chronic histiocytic intervillositis, villitis, chorioamnionitis, and funisitis have been reported in different clinical contexts. Some may represent maternal or fetal immune responses, while others reflect bacterial infection, chronic inflammatory disease, or an unrelated pregnancy complication. Pathology reports should describe the pattern and severity rather than assign causation beyond the available evidence.

The most informative interpretation combines morphology with clinical correlation. A placenta showing infarction in a patient with severe COVID-19 may support a vascular injury hypothesis, but the same lesion in a patient with preeclampsia cannot be attributed confidently to SARS-CoV-2. Standardized sampling and terminology are essential when findings from multiple hospitals or countries are compared.

Linking Tissue Findings To Neonatal Outcomes

Placental pathology can help explain why some fetuses tolerate maternal infection while others develop growth restriction, abnormal fetal monitoring, preterm birth, or neonatal compromise. Reduced maternal perfusion may limit oxygen and nutrient delivery. Extensive fetal vascular malperfusion may impair the functional circulation of the placenta. Inflammation may disrupt the barrier between maternal and fetal compartments.

These mechanisms do not produce identical outcomes in every pregnancy. Gestational age at infection is important because the placenta changes structurally throughout pregnancy. A mild infection late in gestation may have different consequences from severe disease during a period of rapid placental development. Maternal comorbidities, vaccination status, viral variant, treatment, and the reason for delivery also influence neonatal outcomes.

Vertical transmission must be assessed through more than a placental result. A credible investigation may include maternal and neonatal polymerase chain reaction testing, antibody studies, amniotic fluid analysis when available, examination of placental tissue, and clinical evidence of neonatal infection. Timing is critical because contamination during delivery or after birth can complicate interpretation.

A pathological report can therefore support clinical reasoning without replacing it. It may identify a plausible route to fetal compromise, reveal an unexpected inflammatory or vascular lesion, or guide review of maternal and neonatal records. It cannot, by itself, determine whether a newborn’s condition was caused by COVID-19.

Evidence From Multidisciplinary Research

The strongest research has brought together obstetricians, neonatologists, infectious disease specialists, pathologists, epidemiologists, and laboratory scientists. This approach is particularly important because placental findings are vulnerable to selection bias. Early case series often included placentas from severe cases, complicated deliveries, or pregnancies referred for specialized examination, which could make unusual lesions appear more frequent than they are in the broader population.

Control groups are equally important. Comparisons with unaffected pregnancies, pregnancies complicated by other respiratory infections, and pregnancies involving preeclampsia or thrombosis can help identify which changes are distinctive and which reflect general maternal illness. Research should also account for vaccination, prior infection, treatment, maternal age, body mass index, and socioeconomic factors that influence pregnancy outcomes.

The scientific record from international perinatal meetings remains useful for understanding how these questions developed. Archived material connected with the canceled Tokyo congress, including the FAOPS Congress archive, reflects the broader setting of perinatal and neonatal research in which COVID-19 questions became urgent. Although the 2020 event could not proceed because of the pandemic and travel restrictions, the need for collaboration across Asian and Oceania institutions continued through publications, registries, and later scientific meetings.

Building A Reliable Pathology Workflow

A consistent workflow begins before delivery. The clinical team should document the timing and severity of maternal infection, laboratory confirmation, respiratory support, anticoagulation, vaccination, hypertensive disease, fetal growth concerns, and the indication for delivery. This information helps the pathologist prioritize relevant examination and avoid interpreting tissue in isolation.

Gross examination should record placental weight, dimensions, completeness, membranes, cord insertion, cord abnormalities, color changes, firmness, and visible infarcts or hematomas. Representative blocks should include umbilical cord, membranes, maternal and fetal surfaces, normal-appearing parenchyma, and every grossly abnormal area. Additional sampling may be appropriate when fetal growth restriction, stillbirth, severe maternal disease, or suspected infection is present.

Laboratories handling suspected infectious material must follow appropriate biosafety and institutional protocols. Molecular and immunohistochemical testing should use validated methods, suitable controls, and clear reporting language. Terms such as “viral RNA detected” should not be treated as interchangeable with “active placental infection” unless the evidence supports that stronger claim.

Practical recommendations for clinicians and pathology teams include:

• Pair placental findings with maternal, fetal, and neonatal clinical data.
• Use standardized gross descriptions, sampling protocols, and diagnostic terminology.
• Distinguish viral detection from viral replication and confirmed vertical transmission.
• Compare findings with appropriate control groups and relevant maternal comorbidities.
• Record gestational age, vaccination history, treatment, and timing of infection in research datasets.

What Future Studies Should Clarify

Future studies need prospective designs that collect placentas systematically rather than focusing only on severe or unusual cases. Serial maternal assessments, fetal growth surveillance, neonatal testing, and long-term follow-up could show whether particular placental patterns predict outcomes beyond delivery. Such studies would also help separate the effects of infection from those of hospitalization, medication, prematurity, and underlying disease.

More work is needed on the changing biology of the placenta across gestation. Single-cell analysis, spatial transcriptomics, proteomics, and advanced imaging may clarify which placental cells respond to infection and whether inflammatory signals persist after maternal recovery. These approaches can complement routine histology, but they should remain connected to clinically meaningful outcomes.

Pathology also has a role in preparedness for future respiratory outbreaks. A shared framework for specimen collection, tissue preservation, molecular testing, and data reporting would allow hospitals to compare findings rapidly during an emerging epidemic. The goal is a clinically useful evidence base that explains risk without overstating the certainty of any individual placental lesion.

Placental pathology remains one of the clearest ways to examine the biological bridge between maternal COVID-19 and fetal health. When interpreted with rigorous laboratory methods and multidisciplinary clinical information, it can reveal vascular stress, inflammatory activity, and possible infection at the maternal-fetal interface. Perinatal teams can strengthen that knowledge by preserving well-documented specimens, contributing standardized data, and applying findings carefully to counseling and care.