The placenta is an organ that forms in the uterus during pregnancy, serving as a lifeline between the pregnant individual and the developing baby. It provides oxygen and nutrients to the fetus through the umbilical cord. Additionally, the placenta removes waste products and carbon dioxide from the baby’s blood, acting much like the baby’s lungs, kidneys, and liver before birth. This temporary organ is fundamental for supporting a healthy pregnancy.
Where the Placenta Normally Attaches
The placenta typically attaches to an area within the uterus that provides optimal conditions for fetal growth. The fertilized egg implants into the uterine wall, and the placenta develops at this site. Most often, it positions itself at the top (fundus) or on the sides (lateral) of the uterus. These locations are considered normal and advantageous for several reasons.
Placing itself higher in the uterus, away from the cervix, allows for an unobstructed birth canal as pregnancy progresses. The upper and side walls of the uterus generally offer a good blood supply, essential for the efficient transfer of nutrients and oxygen to the baby and the removal of waste. A robust blood supply at the attachment site supports the placenta’s ability to facilitate metabolic exchange. The specific area of implantation sets the stage for the placenta’s position throughout pregnancy.
Common Normal Placenta Locations
Within the normal attachment zone, several common positions for the placenta exist, each considered healthy and not typically posing risks to pregnancy or delivery. These include anterior (front wall, closest to the abdomen), posterior (back wall, nearer to the spine), and fundal (very top of the uterus) positions.
These terms describe where the placenta has implanted and are all normal variations. An anterior placenta acts as a cushion, potentially making it harder to feel fetal movements early in pregnancy, but does not typically cause health problems. A posterior placenta is a common and healthy position that may allow for earlier and stronger perception of fetal movements. Lateral positions, on either side of the uterus, are also normal.
Placental Position Changes
The placenta does not physically “move” within the uterus, but its position can appear to change relative to the cervix as the uterus expands. This phenomenon is often referred to as “placental migration” or “trophotropism.” As the uterus grows and stretches, the area where the placenta is attached also stretches upwards. This upward shift is common, especially for placentas that were initially low-lying.
For example, a placenta close to the cervix in early pregnancy often resolves to a normal position higher in the uterus by the third trimester. This occurs because the placenta tends to grow towards areas of better blood supply, typically in the upper uterus. Around 95% of placentas initially identified as low-lying will move into a safe position for birth by approximately 32 weeks. This natural adjustment frequently alleviates concerns about an initially low-lying placental position.
When Placenta Position Isn’t Normal
While many placental positions are normal, one common deviation is placenta previa. This occurs when the placenta attaches low in the uterus, covering part or all of the cervix (the opening to the vagina). This condition can lead to significant concerns due to the potential for severe bleeding. Bleeding can occur before, during, or after delivery because the cervix thins and opens, causing blood vessels connecting the placenta to the uterus to tear.
Placenta previa prevents a safe vaginal delivery and typically necessitates a cesarean section (C-section) to avoid excessive blood loss. Most cases are diagnosed through ultrasound in the second trimester. While some low-lying placentas resolve, a placenta that completely covers the cervix is less likely to move and often requires a planned C-section.