The placenta is a temporary organ that develops during pregnancy, serving as the interface between the mother and the developing fetus. It transfers oxygen and nutrients from the maternal bloodstream to the baby while removing waste products. Monitoring the placenta’s development and position is a standard part of prenatal care. Ultrasound is the primary tool used to assess this organ throughout gestation, confirming its presence, tracking its maturity, and noting its location.
Initial Visibility: The Early Timeline
The placenta does not instantly appear as a fully formed structure on an ultrasound image. Its initial visibility is tied to the development of the chorion frondosum, which eventually forms the placenta. Early detection relies heavily on transvaginal ultrasound (TVUS), which provides a clearer image of the uterine cavity in the first trimester.
The developing placenta is first visible as a localized thickening or area of increased echogenicity near the gestational sac. This is often detectable as early as eight weeks gestation, where the chorionic frondosum begins to attach to the uterine wall. The organ becomes more clearly defined and distinguishable from surrounding uterine tissue by 10 to 12 weeks. After the first trimester, around 12 to 14 weeks, the placenta is typically well-established, allowing for a clearer assessment of its size and overall structure using a standard transabdominal ultrasound.
Developmental Stages and Changing Appearance
Once the placenta is clearly visible, its appearance on ultrasound changes over time, reflecting its aging and maturation process. Clinicians use the Grannum classification system to grade the placenta from 0 to 3, based on visual changes in the chorionic plate, placental substance, and basal layer. This grading system primarily tracks the progression of calcification.
A Grade 0 placenta, typically seen before 18 weeks, has a smooth, well-defined chorionic plate and a homogeneous internal texture with no signs of calcification. The transition to Grade 1, usually occurring between 18 and 29 weeks, is marked by subtle indentations of the chorionic plate and the appearance of scattered, punctate echogenic areas within the placental tissue. These bright spots represent the first signs of calcification.
A Grade 2 placenta, which often appears after 30 weeks, shows more pronounced, comma-like indentations of the chorionic plate and the beginning of calcification along the basal layer, the side of the placenta attached to the uterus. The final stage, Grade 3, is generally expected near term, around 39 weeks or later. It is characterized by indentations that extend completely from the chorionic plate to the basal layer, dividing the placenta into distinct compartments called cotyledons. When a Grade 3 placenta appears prematurely, such as before 34 weeks, it can sometimes be a sign of underlying issues, prompting closer monitoring.
Why Visibility Matters: Location and Placement
Locating the placenta is a standard part of the mid-pregnancy anatomy scan, typically performed between 18 and 21 weeks. The placenta can attach to any wall of the uterus, with common positions including anterior (front wall), posterior (back wall), fundal (top wall), or lateral (side wall). Anterior or posterior placement is a normal finding and does not affect the health of the baby, although an anterior placenta may cushion fetal movements.
The most clinically relevant aspect of placental location is its relationship to the cervix, the opening of the uterus. A finding of a “low-lying placenta” occurs when the placental edge is situated less than two centimeters away from the internal opening of the cervix. This is a common finding in the second trimester, affecting 10 to 15% of pregnancies at the 20-week scan, but it often resolves as the uterus expands and pulls the placenta upward, away from the cervix.
If the placenta remains low or completely covers the cervix in the third trimester, it is then defined as Placenta Previa. Detection of this condition via ultrasound is important because it can lead to severe bleeding during labor and delivery, necessitating a planned Cesarean section. Therefore, any low-lying finding at the mid-pregnancy scan requires a follow-up ultrasound, often at 32 weeks, to confirm that the placenta has migrated to a safe position for birth.