The placenta is a temporary organ that develops within the uterus during pregnancy, serving as the interface for nutrient and oxygen transfer from the mother to the fetus. It also removes waste products and produces hormones essential for maintaining the pregnancy. When medical professionals refer to a “long placenta,” they are typically describing placentomegaly, where the organ is significantly larger or heavier than expected for the gestational age. An abnormal size suggests an underlying maternal or fetal issue that requires specific medical attention.
How Placentas Are Measured
A placenta is determined to be enlarged based on its physical measurements, specifically its thickness and its weight relative to the pregnancy stage. Placentomegaly refers to a placenta that exceeds the normal size parameters established for a given gestational week. On a prenatal ultrasound, a general guideline suggests that placental thickness in millimeters should roughly correspond to the week of gestation. For example, a placenta measuring 30 millimeters at 30 weeks is generally considered normal.
A placenta is often considered thickened if its measurement exceeds 4 centimeters during the second trimester, or over 6 centimeters in the third trimester. Post-delivery, an enlarged placenta is typically defined by weight, with a normal term placenta weighing around 500 to 600 grams. A weight exceeding 750 grams may be classified as placentomegaly. The size is also evaluated by the fetoplacental weight ratio, which is normally around 1:6, meaning the fetus weighs approximately six times more than the placenta.
Underlying Conditions That Cause Enlargement
Placental enlargement is not a primary disease but rather a physical manifestation of the body’s response to a specific maternal or fetal health challenge. The causes are broadly categorized into maternal conditions and fetal or infectious conditions. The most frequent maternal cause is uncontrolled or poorly managed diabetes mellitus, where high glucose levels prompt the placenta to grow larger through hypertrophy to meet the fetus’s metabolic demands. Severe maternal anemia can also lead to an enlarged placenta, as the organ increases its surface area to compensate for the mother’s reduced oxygen-carrying capacity.
Fetal and infectious conditions represent another significant group of causes. Infections caused by pathogens such as Cytomegalovirus (CMV), syphilis, and rubella, often referred to as TORCH infections, can lead to placental swelling and inflammation. This infectious process triggers an immunological reaction and edema, resulting in a thickened appearance. Another cause is hydrops fetalis, a severe accumulation of fluid in the fetus, which often results in a very large, edematous placenta due to fluid imbalance or heart failure. Chromosomal abnormalities, such as triploidy, and genetic syndromes like Beckwith-Wiedemann syndrome, are also associated with excessive placental growth.
Detection and Ongoing Monitoring
The initial detection of a large placenta usually occurs incidentally during a routine prenatal ultrasound used to assess fetal growth and anatomy. Once placentomegaly is suspected, a more targeted investigation is initiated to determine the root cause. This process includes specific maternal blood tests, such as a glucose tolerance test, to screen for gestational diabetes, which is a common underlying factor. Blood work also screens for infectious agents, including the TORCH panel, to identify any active or recent maternal infection that could be affecting the placenta.
Targeted ultrasounds are performed to gain detailed information about the fetal environment, including the volume of amniotic fluid. Excessive fluid (polyhydramnios) is sometimes associated with a large placenta. Based on the findings, the healthcare team implements a regimen of increased surveillance. This intensified monitoring often includes more frequent non-stress tests (NSTs) and biophysical profiles (BPPs) to closely track fetal well-being. Serial ultrasounds continue to monitor the placenta’s structure and the fetus’s growth pattern, allowing for proactive management of potential complications.
Associated Risks During Pregnancy and Delivery
The presence of placentomegaly carries a set of distinct risks for both the mother and the developing fetus, stemming either from the large size itself or the underlying condition that caused the enlargement. A primary risk to the mother during delivery is postpartum hemorrhage. The enlarged placenta covers a greater surface area of the uterine wall, and after delivery, the uterus must contract tightly to stop the bleeding from this larger wound site. The larger size can also increase the risk of placental abruption, where the placenta prematurely separates from the uterine wall before the baby is born.
For the fetus, the risks are varied and depend heavily on the cause. If the enlargement is due to maternal diabetes, the fetus may experience macrosomia, or excessive growth, leading to complications during birth. Conversely, if the cause is a chronic infection or severe anemia, the placenta may not function efficiently despite its size, leading to intrauterine growth restriction (IUGR) or preterm birth. Both IUGR and preterm birth are associated with higher rates of perinatal mortality and neonatal issues. Timely diagnosis of the underlying cause, whether infectious or metabolic, is important, as addressing the primary issue often allows for better management of these associated risks and improves the overall pregnancy outcome.