Final answer:
Placenta previa is a condition where the placenta covers the cervix partially or fully, identified when the embryo implants near the uterus' internal os. However, as pregnancy progresses, the placenta often migrates away from the cervix toward the fundus, decreasing the risk associated with this condition as the placenta develops fully by weeks 14-16.
Step-by-step explanation:
Placental Migration in Pregnancy
When an embryo implants in a less than optimal location, such as near the internal os of the uterus, a condition known as placenta previa can develop. This occurs when the placenta grows to partially or completely cover the cervix. During early pregnancy, this condition is identified, but as pregnancy progresses, the placenta often appears to 'migrate' upwards towards the fundus of the uterus in many cases, reducing the potential complications associated with placenta previa. The process of the placenta taking over the role of feeding the embryo from the decidual cells is gradual, as the placenta develops from both maternal and fetal tissues, and is usually complete by weeks 14-16 of pregnancy.