Low placenta at the mid trimester anomaly scan-A cause for concern?

Objective: This study was aimed to assess the predictive validity of internal os distance (IOD) in mid-trimester scan for the prediction of third-trimester low-positioned placenta, and to define a cut-off of IOD at which third-trimester low-positioned placenta could be identified, see the association of placental site (anterior/posterior), previous history of cesarean section with abnormal location of placenta in third trimester and see the maternal and neonatal outcomes.

Methods: It was a prospective cohort study of women with low-positioned placenta (IOD < 20 mm) at mid-trimester anomaly scan followed up in third trimester till birth. Relative risks for a low-positioned placenta in the third-trimester were calculated for women with posterior versus anterior, low-lying placenta versus placenta previa and positive versus negative history of cesarean section. Multilevel likelihood ratios and corresponding ROC curves for different ranges of IOD were calculated.

Results: Women with posterior placenta had a high risk of low positioned placenta compared to anterior placenta (9.28% vs. 0.74%); RR 1.45, similarly women with placenta previa had high risk compared to low lying placenta (68.57% vs. 1.69%); RR 6.51, so did the women with previous cesarean section (9.41% vs. 5.87%); RR 1.47.

Conclusions: 93.42% placenta which were low positioned in mid trimester were normally situated in third trimester. The cut-off for IOD in anterior placenta was -40 mm and in posterior placenta was 14.3 mm. IOD measurement and interpretation seems promising.