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Low-lying placenta occurs when the placenta extends into the lower uterine segment and its edge lies too close to the internal os of the cervix, without covering it. The term is usually applied when the placental edge is within 0.5-5.0 cm of the internal cervical os 1. Some alternatively give the term when the placental edge is within 2 cm from the internal cervical os 5.
Our expert advisory panel suggests taking 2-3 cm in mid-second trimester and around 4 cm in third trimester (reference required).
It has also classified under the benign end of the spectrum of type 1 placenta previa, although some restrict the term "previa" only for the situation in which the placenta covers the internal cervical os.
The estimated prevalence may be as high as 10-30% of all pregnancies 2,7. The majority of placentas classified as low-lying in early pregnancy (12-14 weeks) reach a normal position on subsequent scanning later during the pregnancy due to placental trophotropism.
Transvaginal ultrasound is more accurate for evaluation of a low-lying placenta than transabdominal ultrasound 8.
A low-lying placenta detected on ultrasound in the second trimester typically resolves by the mid-third trimester. Rarely (in around 1.6% cases) it can persist to term or near term 9.
Situations that may mimic a low lying placenta on a trans-abdominal scan includes