A low-lying placenta is a finding during pregnancy ultrasounds where the placenta, the organ providing nutrients and oxygen to the developing fetus, attaches to the lower part of the uterus. While this may sound concerning, it is a common occurrence, especially during the middle stages of pregnancy.
What a Low-Lying Placenta Means
The placenta typically attaches to the top or sides of the uterus. A low-lying placenta is diagnosed when its edge is less than 20 millimeters (about 0.8 inches) from the cervix, the opening of the uterus. This positions the placenta close to the birth canal. It is distinct from placenta previa, a more significant condition where the placenta completely covers the cervix. A low-lying placenta is considered less severe than placenta previa.
Prevalence at the 20-Week Scan
A low-lying placenta is a frequent discovery during the routine 20-week anatomy scan. It is detected in approximately 1 in 20 women at this stage, with some estimates suggesting prevalence as high as 10-30% in the second trimester. The 20-week scan is a common time for this finding because the uterus is still growing and expanding, and the placenta’s position relative to the cervix is being assessed. This finding at 20 weeks often resolves on its own as pregnancy progresses.
Monitoring and Management
When a low-lying placenta is identified at the 20-week scan, healthcare providers typically recommend follow-up monitoring. This usually involves another ultrasound scan around 32 weeks of gestation to re-evaluate the placenta’s position. Sometimes, a transvaginal ultrasound, which offers a clearer view, may be used. As the uterus continues to enlarge and stretch, the placenta often appears to “move up” and away from the cervix. This upward shift is not the placenta physically migrating, but rather the lower uterine segment stretching and thinning, pulling the placenta away from the cervical opening.
In the majority of cases, a low-lying placenta diagnosed at 20 weeks resolves naturally before delivery. About 89.9% of placentas clear the cervix by 32 weeks, and 95.9% resolve by 36 weeks. If the placenta has moved sufficiently, usually more than 20 millimeters from the cervix, a vaginal birth may be possible.
When It Persists
In a smaller number of pregnancies, the low-lying placenta does not resolve and remains close to or covers the cervix by late pregnancy, which is then classified as placenta previa. Only about 1.6% of low-lying placentas diagnosed in the second trimester persist to term. If the placenta remains less than 20 millimeters from the cervix or covers it at 36 weeks, a caesarean section is recommended. This is because a vaginal delivery could lead to severe bleeding for the birthing parent and pose risks to the baby.
A persistent low-lying placenta or placenta previa carries an increased risk of vaginal bleeding, particularly in the second half of pregnancy. This bleeding can sometimes be heavy, potentially leading to preterm birth or the need for blood transfusions. Healthcare providers may advise avoiding activities that could trigger bleeding, such as strenuous exercise or sexual intercourse. If bleeding occurs, seek immediate medical attention. Factors that can increase the likelihood of a persistent low-lying placenta or placenta previa include a history of previous caesarean sections, advanced maternal age, and fertility treatments.