The placenta is a temporary organ that develops in the uterus during pregnancy, serving as a bridge between the birthing parent and the developing fetus. It facilitates the exchange of oxygen and nutrients, removes waste products like carbon dioxide, and supports fetal growth. This organ acts as the baby’s lungs, kidneys, and liver until birth. It also produces hormones that support the pregnancy and prepare the body for childbirth.
The Implantation Process
Placenta formation begins shortly after fertilization when the fertilized egg, now a blastocyst, travels to the uterus. Implantation, the process where the blastocyst attaches to the uterine wall, occurs about six to ten days after fertilization. This attachment happens in the upper part of the uterus, often in the posterior or anterior wall.
During implantation, the outer layer of the blastocyst, called the trophoblast, interacts with the uterine lining, the endometrium. Trophoblast cells burrow into the endometrium, establishing the initial connection that develops into the placenta. The uterine environment must be receptive for this process to succeed.
Influencing Factors
Several factors can influence where the placenta implants within the uterus. The initial site where the embryo attaches to the uterine wall is a primary determinant, and while often in the upper uterus, the exact spot can vary.
The condition of the uterine lining also plays a role. Scars from previous uterine surgeries, such as Cesarean sections or dilation and curettage (D&C) procedures, can influence implantation location. For instance, a history of Cesarean sections increases the risk of the placenta implanting in the lower part of the uterus, over or near the prior scar. Uterine anomalies or fibroids, which are non-cancerous growths, can also affect available implantation sites and the placenta’s final position.
Varied Placenta Positions
The placenta can develop in various locations within the uterus, identified during routine ultrasound scans.
- Fundal: at the top of the uterus.
- Anterior: on the front wall closer to the abdomen.
- Posterior: on the back wall closer to the spine.
- Lateral: on the left or right side.
An anterior placenta can make it harder to feel fetal movements initially due to its cushioning effect.
The placenta may implant lower in the uterus. A “low-lying placenta” is when its edge is close to the cervix but does not cover it. If the placenta partially or completely covers the cervical opening, it is termed “placenta previa.” Types include marginal (at the edge of the cervix), partial (partially covering), and complete or total (fully covering).
Managing Placenta Position
The position of the placenta is assessed during a routine anatomy ultrasound, performed between 18 and 21 weeks of pregnancy. If a low-lying placenta or placenta previa is identified early, the placenta may “migrate” upwards and away from the cervix as the uterus grows and stretches. Many cases of low-lying placenta resolve by the third trimester.
If the placenta remains low-lying or is diagnosed as placenta previa later in pregnancy, additional monitoring with ultrasounds is recommended. For placenta previa, a Cesarean section is the safest delivery method to prevent severe bleeding, as a vaginal birth could cause the placenta to detach. Delivery for placenta previa is planned between 36 and 37 weeks of gestation in stable cases. Patients are advised to report any vaginal bleeding immediately.