Low-lying placenta: who should be recalled for a follow-up scan?

Introduction: The incidence of low-lying placenta or placenta praevia in the second trimester has been reported at 1-5%; however, recent unpublished audits suggest our recall rates are higher. We wanted to assess our recall rates in a large sample size and determine whether we could reduce the placenta-os distance for recalling women with low-lying placenta, while still identifying all cases of placenta praevia at delivery.

Methods: We undertook a retrospective analysis from March 2005 to March 2008 of women attending for 18-20-week obstetric ultrasounds. Patients with a singleton pregnancy and a placenta ≤2 cm from the internal cervical os were included. Follow-up scan results and delivery data were collected.

Results: Four hundred eight women were identified as having a low-lying placenta at the 18-20-week scan (107 (9%) at Dunedin Hospital and 301 (5%) at Otago Radiology). Fifty-eight women (14%) were excluded, leaving 350 women included in the analysis. Three hundred seventeen (91%) had a placenta clear of the internal os on their follow-up scan while 33 women (9%) had persistent placenta praevia. At a distance of ≥1.9 cm, there was 100% sensitivity for detection of placenta praevia on the 18-20-week scan. As the placenta-os distance decreases the sensitivity for detection of placenta praevia reduces.

Conclusions: Placenta praevia at term can occur where the placenta is up to 1.9 cm from the internal cervical os on the 18-20-week anatomy scan. Consequently, we will continue to recall women for a follow-up scan where the placenta is ≤2 cm from the internal os.