The placenta is a temporary organ that develops during pregnancy, transferring oxygen and nutrients from the mother’s bloodstream to the baby and removing waste products. The location where this organ implants within the uterus is important information for prenatal care providers. Ultrasound, a non-invasive imaging technique, serves as the primary method for assessing and documenting the placenta’s position. This routine assessment helps ensure a safe pregnancy progression and delivery plan. Understanding the terminology and visual findings on an ultrasound report provides clarity during pregnancy.
Understanding the Standard Placental Positions
The terminology used to describe placental placement refers to the wall of the uterus where the organ has attached. This location is determined by where the fertilized egg initially implanted.
An anterior placenta is situated on the front wall of the uterus, positioned toward the mother’s abdomen. Conversely, a posterior placenta attaches to the back wall, lying closer to the mother’s spine. Both anterior and posterior placements are considered normal variations and are the most frequent positions observed.
A fundal placenta is located at the very top of the uterus (the fundus). The placenta may also attach to either the left or right side, described as a lateral placenta. It is common for the placenta to span multiple areas, such as being described as antero-fundal or postero-lateral. These descriptive terms generally do not affect the health of the pregnancy.
Interpreting the Placenta on the Ultrasound Screen
A sonographer must accurately identify the placenta and map its boundaries relative to the uterus. On the ultrasound screen, the placenta appears as a uniformly textured, solid structure along the uterine wall. This appearance is described as echogenic, meaning it reflects sound waves well, making it look brighter than the surrounding amniotic fluid.
The texture of a normal placenta is generally homogeneous, indicating a consistent appearance throughout the tissue. It is distinct from the uterine muscle (myometrium), which appears less reflective. The placenta is separated from the myometrium by a thin, dark area called the retroplacental clear space. The placenta may also contain small, dark, fluid-filled spaces called venous lakes, which are normal findings representing slow-flowing blood.
To determine the exact position, the sonographer uses the cervix and the internal os (the opening into the uterus) as primary landmarks. The entire uterus must be scanned in both the transverse (side-to-side) and sagittal (top-to-bottom) planes to map the full extent of the placenta. This comprehensive view ensures the entire placental edge is located and its relationship to the internal os is properly assessed.
The precise location of the placental edge is measured by calculating the distance between the edge and the internal os. This measurement is typically taken using a transvaginal ultrasound, which is the most accurate method for assessing the lower uterine segment. The measurement determines if the placenta is safely positioned away from the birth canal or if it is low-lying, requiring closer monitoring.
When Placental Location Requires Monitoring
While most placental positions are normal, the lower edge is assessed to check for placements that could affect delivery. A low-lying placenta is diagnosed when the placental edge is located within 20 millimeters of the internal cervical os, but does not cover it. This finding is common during the routine second-trimester anatomy scan around 20 weeks.
If the placental tissue completely or partially covers the internal os, the condition is termed placenta previa. This placement is classified as complete, partial, or marginal depending on the degree of coverage, and it carries an increased risk of bleeding later in the pregnancy. Both placenta previa and a low-lying placenta require monitoring because the lower part of the uterus stretches as the pregnancy progresses.
This stretching often causes the placenta’s position to change, a phenomenon sometimes called “placental migration.” For most women with a low-lying placenta in the second trimester, the issue resolves by the third trimester. If the placenta remains low at 32 weeks, an additional follow-up scan is scheduled around 36 weeks to determine the final placement before delivery. If the placenta is still covering the os or is less than 20 millimeters away at that stage, a cesarean delivery is recommended to avoid complications.