Being told a “placental lake” is present on an ultrasound can be alarming for any expectant parent. The immediate concern is whether this discovery indicates a danger to the developing fetus or the pregnancy itself. In the vast majority of cases, however, a placental lake is an incidental and normal variation in placental structure. These findings are common during routine ultrasound screening and rarely impact the health of the baby or the outcome of the birth. Understanding what placental lakes are and how they are managed by medical professionals can help alleviate anxiety.
Understanding What Placental Lakes Are
Placental lakes are enlarged spaces within the placenta filled with maternal blood. These areas are part of the intervillous spaces, which are normal placental structures responsible for nutrient and oxygen exchange between the mother and fetus. The term “lake” refers to the appearance of these spaces when they are larger than usual.
On an ultrasound, placental lakes appear as well-defined, anechoic areas, meaning they look black because they do not reflect sound waves. They consist primarily of pools of maternal venous blood, which may show a slow, swirling flow. They are typically smooth-walled and compressible, characteristics that help differentiate them from more serious placental pathologies.
It is important to distinguish a placental lake from a placental lacuna. Placental lacunae are irregular, turbulent spaces commonly associated with the placenta accreta spectrum, a condition where the placenta invades the uterine wall. Isolated placental lakes, by contrast, are considered a variation of normal anatomy and are not linked to this invasive condition. They are found in approximately 20 to 30% of all pregnancies, making them a frequent finding on mid-pregnancy scans.
Common Outcomes and Absence of Risk
The finding of small placental lakes is considered a benign occurrence with no adverse effect on the pregnancy. They are so common that sonographers often do not report them unless they are large, multiple, or accompanied by other complicating factors. Research consistently shows that the mere presence of a placental lake is not associated with a higher risk of complications.
Studies evaluating pregnancy outcomes have found no correlation between isolated placental lakes and adverse events. These include low birth weight, premature birth, preeclampsia, fetal growth restriction (FGR), or placental abruption. The presence of these incidental findings does not indicate placental dysfunction or compromise the baby’s well-being.
Most placental lakes are small and stable, often appearing later in the pregnancy and remaining stable or resolving entirely without intervention. Even when considering the size and number of lakes, studies have found no association with poor outcomes. The typical prognosis is excellent, and these structures generally do not require a change in routine prenatal care.
Tracking and Management
The finding of an isolated placental lake does not typically warrant increased medical surveillance; standard prenatal care is usually maintained. Management focuses on monitoring the overall health and growth of the fetus, not treating the lake itself, as it is not considered a disease. Healthcare providers continue with routine ultrasound assessments to ensure the baby is developing appropriately.
Increased monitoring, such as follow-up ultrasounds, may be recommended only when the placental lakes present with specific characteristics or are accompanied by other high-risk factors. Closer attention may be prompted if the lakes are extremely large (greater than five centimeters) or if multiple lakes are found very early in the second trimester (before 20 weeks gestation).
Another factor prompting closer attention is the presence of placental lakes alongside a significantly thickened placenta or other markers of placental insufficiency. If surveillance is increased, it usually involves serial growth scans to track the baby’s size and well-being. Doppler studies may also be used to assess blood flow in the umbilical artery, indicating placental efficiency. This monitoring is guided by the risk the lakes pose in combination with other factors, not by the isolated finding of a common placental variation.