The ADC map derived from MR DWI imaging can quantify molecular water diffusion throughout fetal development and may be able to identify placental pathology, such as infarct, when it exists.¹ The purpose of this research is to establish normal ADC values for the healthy placenta and investigate the relationship between placental ADC values and distance from umbilical cord insertion.This HIPAA-compliant retrospective study was approved by our Institutional Review Board. 78 normal singleton pregnancies between 20 and 41 weeks gestational age (mean 27.5, range 19.9-37.3, SD of 5.1) were included in the study. MR DWI images were acquired on a 1.5 Tesla system (Siemens Sonata, Siemens Medical Solutions USA, Malvern, PA). A 4-channel body array coil was used for signal reception. The optimized DWI protocol consists of a single-shot, spin-echo echo planar imaging (EPI) sequence with TR/TE of 3100/97 ms; in plane resolution of 1.9 x1.9 mm2, square field of view (FOV) of 240 mm; 128 × 128 matrix, and 5-mm contiguous slices with three excitations. Three orthogonal gradients were applied with a b value of 1,000 s/mm². HASTE and TRUE-FISP sequences were also used to acquire anatomic images of the placenta without breath hold. ADC maps were created using assigned b-values. The average ADC (ADC2D) of the entire cross section of the placenta at the level of the umbilical cord insertion was measured in the plane that provided the maximum cross-sectional area. Additional ROIs were defined in the same plane on the ADC maps at distances of 1.5 cm (ADC1.5), 3 cm (ADC3), 6 cm (ADC6), and 9 cm (ADC9) from the point of the umbilical cord insertion (Figure 1). Each ROI was 2 cm2. Inter- and intra-examiner repeatability and reproducibility were evaluated using intraclass correlation coefficients (ICC) and Bland–Altman plots. The ADC values of individual ROIs vs. the distance from the insertion of the umbilical cord were plotted individually for each placenta to look for a correlation between the ADC values and the distance from the insertion of the umbilical cord. The average ADC of the cross section of the placenta, and these from the ROIs at 1.5 cm, 3.0 cm, and 6.0 cm for all placentas were compared using a standard ANOVA test. p<0.05 was considered as significant.ADC values decreased slightly as a function of gestational age (GA) (ADC (x10^-5mm²/s) = 205.9385-0.9189 x GA, SD = 27.692, R₂ = 0.03) (Figure 2). There was no statistically significant difference in mean ADC values between samples taken from the entire cross-section of the placenta or single 2 cm² samples at 1.5, 3, or 6 cm from the umbilical cord insertion shown in Figure 3 (ANOVA, F=0.18, p= 0.91). Intra- and interobserver ICC for measurements obtained from the entire cross-section of the placenta were 0.97 and 0.963, respectively. Inter- and intraobserver limits of agreement on Bland-Altman plots were -17.6 to 24.7 x10^-5 mm²/s) and -21.9 to 20.3 x10^-5 mm²/s, respectively.Several previous DWI studies have suggested that DWI can play an important role for early detection of placental based disorders. Our results show the statistical similarity between the average ADC values of the cross-section of the placenta and smaller ROIs taken at varying distance from the insertion of the umbilindicates that the ADC throughout the placenta is homogeneous at high b-values (1,000 mm²/s). A previous study suggests that DWI may be sensitive to a variety of placental pathology, including hemorrhage, thrombi, ischemia, chorioamnionitis, and fibrin deposition.² To our knowledge, this is the first MRI study of the placenta to evaluate the spatial function using MRI diffusion weighted imaging.ADC values of a healthy placenta decrease slightly with gestational age. The lack of heterogeneity from samples obtained from different regions of the placenta as well as good intra- and interobserver repeatability and reproducibility suggests that diffusion quantitation can be confidently performed throughout the placenta.