Background There is an increasing incidence of placental invasion and MRI is frequently used as a diagnostic tool. Three MRI features have been identified (heterogeneity, placental bands and uterine bulging) as indicators of invasion. However, little research has been carried out to determine whether these features are specific to placental invasion or if they also occur in the normal placenta.

Aim To determine whether the features of placental invasion on MRI are present in normal placentas.

Method 200 fetal MR scans were assessed retrospectively for the three MRI features of placental invasion.

Results Of 200 MRI scans assessed 63% were from women in their 2^nd trimester and 37% from women in their 3^rd trimester. The mean GA was 172 (range: 126-252 days). There was no evidence of uterine bulging in any of the images. Where dark intraplacental bands were present (33% n=66) more had diffuse bands in the 3^rd trimester (28%) than the 2^nd (9%), and more had localised banding in the 2^nd (19%) than the 3^rd trimester (14%). Of the heterogeneous placentas (40.5% n= 81) 42% of the 3^rd trimesters and 18% of the 2^nd showed diffuse heterogeneity. Localised heterogeneity was seen in 12% of the 3rd trimesters and 20% of the 2^nd.

Conclusion Uterine bulging appears to be exclusive to the invaded placenta. However, heterogeneity and intraplacental banding occur in normal placentas. Further research into the nature of the heterogeneity and banding should ensue, to see if there is a difference in the normal and invading placenta.