The term placenta previa refers to abnormally low implantation of the placenta at or over the internal cervical os that precludes vaginal delivery. The exact etiology of placenta previa is unknown. The condition may be multifactorial and is postulated to be related to multiparity, multiple gestations, advanced maternal age, previous cesarean delivery, previous abortion, and possibly, smoking.

The classic presentation of placenta previa is painless vaginal bleeding.

Nearly two thirds of symptomatic patients present before 36 weeks\' gestation, with half of these patients presenting before 30 weeks\' gestation. Transvaginal ultrasound not only assists in diagnosis of placenta location, but can measure the exact distance from the placental edge to the internal os.  

Four categories of abnormality in placental location have been defined:

(1) complete placenta previa, when the placenta covers the internal os completely (may be asymmetric with only a portion of the placenta crossing the os)

(2) partial placenta previa, when the placenta partially covers the internal os

(3) marginal placenta previa, when the placental edge just reaches the margin of the internal os and

(4) low placenta, when the placental edge does not reach, but is within 2 cm of the internal   os.

It is helpful to diagnose not only placenta previa but also low placenta before delivery, because the potential for uterine atony and hemorrhage exists with any placenta in the LUS.

Various predictors have been described to determine the likelihood of persistence of previa at delivery.

- The first predictor is the degree of overlap of the placental edge over the internal os.

- A second predictor is placental edge thickness: those who have thin placental edge, measuring less than 1 cm in thickness or presenting at an angle less than 45°, have a significantly higher rate of vaginal delivery; whereas those who have a thick placental edge have increased rates of emergent cesarean deliveries, hemorrhage, and preterm delivery.

- A third predictor is the rate of apparent placental migration: those patients requiring cesarean for complications attributable to previa have a mean rate of placental migration of 0.3 mm/week, versus 5.4 mm/week for those who have a vaginal delivery or cesarean for other indication.

Pitfalls in the diagnosis of placenta previa -  

- An overdistended bladder compressing the anterior and posterior myometrium, resulting in the placental margin appearing adjacent to the internal os, when the margin is actually removed from the cervix. Thus, examination should be repeated with an empty bladder.

- Another pitfall involves imaging the LUS during uterine contraction. This should be suspected when the myometrium appears thick (greater than 1.5 cm) in the LUS. Therefore, it is important to wait to image the potential placenta previa until after the contraction has resolved.

MR imaging shows the placenta in a gravid uterus as moderately hyperintense on T2-weighted images and allows differentiation between the placenta and the hypointense myometrium. Images obtained perpendicular to the boundary plane between the placenta and myometrium may be most useful for evaluation of the depth of placental invasion into the myometrium. A sagittal T2-weighted image oriented in the plane of the cervix can allow assessment of the relationship between the placental edge and the internal cervical os.