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

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incidence of low-lying placenta or placenta praevia in the second trimester has been reported at 1-5%; however, recent unpublished audits suggest our recall rates are higher.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.

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.

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.

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.