In most pregnancies, the placenta attaches to the front or back of the womb (the uterus). But sometimes it attaches lower down and may cover a part or all of the cervix (the entrance to the womb).
A low-lying placenta is a placenta with a leading edge that is within 2 cm of the opening of the cervix. The place where the cervix opens into the uterus is known as the internal cervical OS. This diagnosis is made by measuring the distance between the edge of the placenta and the internal os on ultrasound (usually a transvaginal ultrasound).
In most cases of a low-lying placenta, the placenta moves upwards and out of the way as the uterus grows during pregnancy. But sometimes the placenta stays in the lower part of the uterus as the pregnancy continues.