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.