The human placenta shows marked developmental changes over the course of pregnancy and previous data suggest that the transport function of the syncytiotrophoblast also changes from first trimester to term. Directional transport of solutes across the placenta depends on asymmetric distribution of transporter proteins in the maternal facing microvillous (MVM) and the fetal facing basal (BM) plasma membrane of the syncytiotrophoblast. It is evident that some solute transport proteins show polarised localisation to the MVM or BM at term, but little is known about the development of polarity throughout gestation. The localisation to MVM and BM of several transport proteins (plasma membrane Ca²⁺-ATPase PMCA, GLUT1, Na⁺-H⁺ exchanger NHE1, transferrin) in term (T) and first trimester (FT) placenta was assessed by immunohistochemistry. Fragments of placental villi were randomly sampled from FT (6-12 weeks; n=4) and T (n=3 or 4) placentas and immediately preserved in zinc fixative (Johannson et al. 2000). Villi were paraffin embedded, sectioned, incubated with primary then biotinylated secondary antibodies and counterstained with methyl green. Six sections were imaged for each placenta and assessed by three independent observers, blind to antibody and gestation. For each placenta, the mean of the observer scores for each antibody was calculated to achieve an average for each placenta. The values are expressed as mean±SEM with n = number of placentas. All the transport proteins investigated were detected in both MVM and BM in both FT and T tissue. Transferrin showed more staining localized to the MVM compared to the BM in both FT (0.93±0.07 compared to 0.26±0.11) and T (1.0±0.01 compared to 0.32±0.13). PMCA (MVM 0.88±0.11 compared to BM 0.90±0.09) and NHE 1 (MVM 0.93±0.04 compared to BM 0.81±0.13) showed equal distribution to the MVM and the BM in FT. At term, PMCA (MVM 0.19±0.10 compared to BM 0.98±0.019) and NHE 1 (MVM 0.65±0.01 compared to BM 1.00±0.01) showed a more polarised distribution to the BM. GLUT1 was expressed equally in the MVM and BM at both FT (MVM 0.74±0.14 compared to BM 0.75±0.15) and T (MVM, 0.75±0.10 compared to BM 0.94±0.02). In conclusion, the non polarised distribution of GLUT1 and the polarised distribution of transferrin was maintained throughout gestation. There was a maturation of the distribution of PMCA and NHE1 from expression on both MVM and BM in FT to predominantly BM at T. The term distribution of NHE1 and PMCA predominantly to BM is consistent with previous reports (Speake et al. 2005). The maturation of distribution of these transporters may have functional implications for ion transport across the placenta during gestation highlighting the need for studies on the mechanisms of transporter trafficking in the human placenta.