A low-lying placenta occurs when it extends into the lower uterine segment, and its edge lies close to the internal os of the cervix without covering it.

Terminology

The term should be used when the edge of the placenta is <2 cm from the cervical internal os in pregnancies >16 weeks gestation ^1,2. A placental edge >2 cm from the os is labelled normal, and a placenta covering the os is placenta praevia ¹.

Epidemiology

The estimated prevalence may be as high as 10-30% of all pregnancies ^3,7. 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.

Risk Factors for Low-Lying Placenta

• previous caesarean section

• advanced maternal age

• multiparity

• smoking

• use of assisted reproductive technologies (ART)

Radiographic features

Ultrasound

Transvaginal ultrasound is more accurate for evaluating a low-lying placenta than transabdominal ultrasound and is safe ^1,8.

Treatment and prognosis

A low-lying placenta detected on ultrasound in the second trimester typically resolves by the mid-third trimester, as such a follow-up ultrasound is recommended at 32 weeks ¹. Rarely (in ~1.5% cases) it can persist to term or near term ⁹.

Differential diagnosis

Situations that may mimic a low lying placenta on a trans-abdominal scan includes

• focal myometrial contraction(s) stretching the lower uterine segment

• overdistended bladder stretching the lower uterine segment

• marginal placenta praevia: especially if the placenta is posterior

• Retroplacental haematoma, which may appear as a hypoechoic or heterogeneous area between the placenta and the myometrium.

• vasa praevia, where fetal blood vessels run over or near the internal cervical os, potentially mistaken for placental tissue.

• placenta accreta spectrum, particularly in cases where the placenta is abnormally adherent to the myometrium and extends into the lower uterine segment.