The placenta is the temporary organ that sustains a pregnancy, acting as the interface for nutrient, gas, and waste exchange between the mother and the developing fetus. Variations in its appearance are a common finding during routine prenatal screening. One such variation is the presence of a placental lake, which is often considered a normal part of placental development. The discovery of these structures during an ultrasound often leads to questions about their nature and effect on the pregnancy. This article aims to provide a clear understanding of what a placental lake is and whether its presence warrants concern.
Defining the Placental Lake
A placental lake is a pool of maternal blood that forms within the substance of the placenta. These structures are defined as enlarged, dilated spaces within the intervillous space, which is the area between the chorionic villi where maternal blood naturally circulates. The lake contains strictly maternal blood, not a mixture of maternal and fetal blood, as the placental barrier separates the two circulatory systems. This finding is common, occurring in approximately 20 to 30 percent of all pregnancies.
On an ultrasound scan, a placental lake appears as a well-demarcated, fluid-filled space. Because fluid does not reflect sound waves, the structure appears nearly black, or anechoic, against the brighter placental tissue. Lakes can be found in various locations, including the subchorionic area near the fetal surface or deeper within the placental tissue (parenchyma). Their size and shape can vary considerably from one pregnancy to the next.
Physiological Formation and Ultrasound Identification
The formation of placental lakes is tied to the process of how the maternal blood supply is established within the placenta. As pregnancy progresses, maternal blood begins to flow freely into the intervillous spaces, starting near the end of the first trimester. This flow creates areas where blood may pool and slow down, leading to the formation of these dilated, blood-filled spaces. Some theories suggest this pooling is related to how the maternal blood vessels, called spiral arteries, remodel themselves to accommodate the increased blood flow of pregnancy.
Lakes are most frequently identified during routine second or third-trimester ultrasound examinations. They are identified by their characteristic appearance as a sonolucent area, meaning they are dark and homogeneous on the gray-scale image. Color Doppler imaging is often used to confirm the structure’s nature by assessing the blood flow within it.
A true placental lake typically shows slow, low-velocity flow or no discernible flow, consistent with pooled venous blood. This flow pattern helps distinguish a benign lake from other, more concerning findings, such as a hematoma or placental lacunae. Placental lacunae, which can be associated with placenta accreta spectrum, often display irregular shapes and a high-velocity, turbulent flow pattern. Differentiating these vascular structures is a major factor in determining appropriate monitoring.
Clinical Significance and Prenatal Monitoring
The medical consensus is reassuring: an isolated, typical placental lake is considered a benign, incidental finding that does not usually affect the health of the mother or baby. Most patients with this finding will continue with standard prenatal care and experience no complications.
The size, number, and timing of the finding can influence the need for closer observation. In rare instances, multiple lakes or a single very large placental lake may prompt additional consideration. While small lakes are generally ignored, a lake measuring greater than two centimeters in diameter, or larger than five centimeters in its maximum length, is sometimes classified as a large lake.
When larger or multiple lakes are identified, particularly if they appear early in the pregnancy, they have been associated with conditions such as the fetus being small for gestational age or fetal growth restriction. This association suggests the lake’s presence may be a sign of altered placental development or function, rather than the cause of the condition.
In these specific scenarios, monitoring shifts to closer surveillance to ensure optimal fetal well-being. This typically involves serial ultrasound examinations to track fetal growth, monitor amniotic fluid, and use Doppler studies to check blood flow in the umbilical cord and other fetal vessels. The primary goal of this focused monitoring is to detect any potential impact on the fetus early enough to intervene if necessary.