Routine third trimester control ultrasound examination for low-lying or marginal placentas diagnosed at mid-pregnancy: is this indicated?

Objective: To determine what proportion of placentas described as low lying or marginal at the mid-pregnancy ultrasound examination are still so described in the third trimester, necessitating delivery by Caesarean section.

Methods: A retrospective chart review of all women delivering at the Centre Hospitalier Universitaire de Sherbrooke (CHUS) from April 1, 2009, to March 31, 2011, was undertaken, and placental location at the mid-pregnancy ultrasound examination was noted. For all cases in which the placenta was described as previa (complete, partial, marginal, or low lying), the control third trimester ultrasound examination, when performed, was revised and so was the mode of delivery, vaginal or Casearean section.

Results: During the study period, 5618 women delivered at the CHUS, and 4884 (86.9%) of these women had an ultrasound examination performed at the CHUS at mid-pregnancy. The placenta was described as low lying or marginal in 412 cases (8.4%). A third trimester control examination was performed in 376 cases (91.2%), and the placenta was still described as low lying or marginal in six cases (1.5%). Four of these 412 women (0.9%) had Caesarean sections for placental reasons.

Conclusion: The vast majority (98.5%) of women with low-lying or marginal placentas at the mid-pregnancy ultrasound examination had normally located placentas in the third trimester, and less than 1% of these women had a Caesarean section for reasons that could be associated with the placental location.