The placenta is an organ that develops within the uterus during pregnancy, serving as a temporary connection between the pregnant individual and the developing fetus. Its primary role involves facilitating the exchange of oxygen, nutrients, and antibodies from the parent’s bloodstream to the baby, while also removing waste products like carbon dioxide from the baby’s blood. This organ acts as the baby’s temporary lungs, kidneys, and liver until birth. Ultrasound technology offers a non-invasive method for monitoring the health and development of both the fetus and the placenta throughout pregnancy. Through sound waves, ultrasound creates detailed images, allowing healthcare providers to assess pregnancy.
Identifying the Placenta
On an ultrasound image, the placenta appears as a distinct, solid structure along the uterine wall. It presents as a uniformly echogenic, or intermediate gray, area. This means it has a consistent texture that stands out from the surrounding amniotic fluid and the uterine muscle. A deep, less reflective band, known as the retroplacental hypoechoic band, separates the placenta from the uterine wall, which is important for identifying healthy implantation.
The placenta’s shape is discoid or flattened-circular, increasing in size and echogenicity as the pregnancy progresses. It is connected to the developing baby by the umbilical cord, which is often seen originating from the placental bulk on ultrasound. The appearance of the placenta may include small anechoic (fluid-filled) areas, representing venous lakes, which are normal variations. By the end of 20 weeks of gestation, the placenta reaches its final thickness and overall shape.
Common Placental Positions
The placenta can attach to various locations within the uterus, and these positions are identified during ultrasound examinations. An anterior placenta implants on the front wall of the uterus, closer to the pregnant individual’s abdomen. A posterior placenta is on the back wall of the uterus, towards the spine. Both anterior and posterior positions are common in many pregnancies.
A fundal placenta is at the top of the uterus, known as the fundus. Lateral placentas attach to the side walls, either left or right. Though less common than anterior or posterior placements, lateral positions are normal variations. The specific position of the placenta is determined by where the fertilized egg initially implants into the uterine wall.
Understanding Specific Placental Locations
Lower placental implantation can have specific implications. A “low-lying placenta” is diagnosed when its lower edge is 0-20 millimeters from the internal cervical opening. It is often identified during the routine 18-to-22-week ultrasound scan. In many cases, a placenta initially identified as low-lying will “migrate” upwards and away from the cervix as the uterus expands with the growing pregnancy.
Placenta previa occurs when the placenta completely or partially covers the internal cervical opening. This obstructs the baby’s typical exit pathway during vaginal birth. It is usually confirmed after 32 weeks, as earlier diagnoses often resolve. The distinction between a low-lying placenta and placenta previa is determined by the precise measurement of the placenta’s edge relative to the cervix, often requiring a transvaginal ultrasound for accurate assessment.