Objective The antenatal detection of small for gestational age (SGA) pregnancies is a challenge, which may be improved by placental MRI. The longitudinal relaxation time (T1) is a tissue constant related to tissue morphology and tissue oxygenation, thereby placental T1 may be related to placental function. The aim of this study is to investigate placental T1 in appropriate for gestational age (AGA) and SGA pregnancies. Methods A total of 132 singleton pregnancies were retrieved from our MRI research database. MRI and ultrasound estimated fetal weight (EFW) was performed at gestational week 20.6-41.7 in a 1.5 T system. SGA was defined as BW≤-15% of the expected for gestational age (≤10th centile). A subgroup of SGA pregnancies underwent postnatal placental histological examination (PHE) and abnormal PHE was defined as vascular malperfusion. The placental T1 values were converted into Z-scores adjusted for gestational age at MRI. The predictive performance of placental T1 and EFW was compared by receiver operating curves (ROC). Results In AGA pregnancies, placental T1 showed a negative linear correlation with gestational age (r=-0.36, p=0.004) Placental T1 was significantly reduced in SGA pregnancies (mean Z-score=-0.34) when compared to AGA pregnancies, p=0.03. Among SGA pregnancies placental T1 was not reduced in cases with abnormal PHE, p=0.84. The predictive performance of EFW (AUC=0.84, 95% CI, 0.77-0.91) was significantly stronger than placental T1 (AUC=0.62, 95% CI, 0.52-0.72) (p=0.002). Discussion A low placental T1 relaxation time is associated with SGA at birth. However, the predictive performance of placental T1 is not as strong as EFW.