The placenta is a temporary organ that develops inside the uterus, serving as the interface between the mother and the fetus. Its primary function is to facilitate the transfer of oxygen and nutrients from the mother’s bloodstream to the growing baby while removing waste products like carbon dioxide. The location where this organ attaches to the uterine wall can vary significantly, determined by where the fertilized egg initially implants. While placental position is routine information collected during prenatal care, its placement cannot be determined by any sensation the mother experiences.
How Medical Professionals Determine Position
The only reliable way to identify the placenta’s location is through medical imaging performed by a healthcare professional. This placement is typically assessed and documented during the routine anatomy scan, generally scheduled between 18 and 21 weeks of gestation. Self-diagnosis is impossible because the placenta is an internal organ attached deeply to the uterine wall, cushioned by layers of muscle and tissue.
The imaging procedure allows the technician to map the exact relationship between the placenta and the rest of the uterus. This assessment details which wall of the uterus the placenta has adhered to, and its proximity to the cervix. Knowing the precise location helps providers prepare for any potential monitoring or adjustments to the delivery plan, though most positions are entirely normal variations.
What Different Placental Positions Mean
Placental position is described by which wall of the uterus the organ has implanted upon. An anterior placenta is positioned on the front wall of the uterus, lying closer to the mother’s abdomen. This placement is a common variation, but the organ acts as a cushion between the baby and the abdominal surface.
Anterior placement can make a mother feel fetal movements later in the pregnancy, or feel them more softly, because the placenta muffles the kicks. Conversely, a posterior placenta is attached to the back wall of the uterus, closer to the mother’s spine. With this position, movements are felt more distinctly and earlier in the second trimester.
A fundal placenta is located high up on the top wall of the uterus. The placenta may also be described as lateral, meaning it is attached to the side walls of the uterus. In the majority of pregnancies, these positions—anterior, posterior, or fundal—do not affect the health of the pregnancy or the ability to have a vaginal delivery.
When Location Needs Special Attention
While most positions are benign, attention is required when the placenta is low in the uterus, near the cervix. A low-lying placenta is diagnosed when the edge of the organ is within two centimeters of the internal opening of the cervix but does not cover it. A more significant concern is Placenta Previa, which occurs when the placenta partially or completely covers the cervical opening.
Placenta Previa poses a risk of heavy bleeding, especially in the third trimester, because the lower uterine segment stretches as labor approaches. If this condition persists, a cesarean section is necessary to prevent severe hemorrhaging that could be life-threatening to both mother and baby. Placenta Previa occurs in about 1 in 200 third-trimester pregnancies.
However, a low-lying placenta discovered during the mid-pregnancy scan often resolves itself due to uterine expansion. As the uterus expands rapidly during the second and third trimesters, the attachment site is carried upward and away from the cervix. Approximately 90% of low-lying placentas identified early in the second trimester will have moved into a safe position by the third trimester.
To confirm that the position has resolved, a follow-up ultrasound is typically scheduled around 32 weeks of gestation. If the placenta is still low at that time, another scan may be performed near 36 weeks. An anterior low-lying placenta has been observed to migrate more frequently than a posterior one, but monitoring is necessary for any low-lying position.