The placenta is unique in that it has low levels of genome-wide DNA methylation compared to other human tissues. Furthermore, overall genome CpG methylation has been observed to increase during gestation. It has also been reported that differential methylation in the placenta occurs in women with preeclampsia compared to those with uncomplicated pregnancies. In this study, we set out to determine the changes in DNA methylation that occur across gestation and if DNA methylation can predict the gestational age of a placenta. We assembled a large data set of publicly available placental DNA methylation data. In total, we had data for 387 placental samples from 8-42 weeks’ gestation from 12 data sets quantified on Illumina Infinium HumanMethylation 27k and 450k arrays. We identified 62 CpG sites that accurately predict the gestational age of the placenta from uncomplicated pregnancies. There was a positive correlation with 27 CpG sites that became hypermethylated with increasing gestational age. Conversely, 35 CpG sites were negatively correlated, and became hypomethylated, with increasing gestational age. In addition, the 62 CpG sites are associated with genes that are known to have a critical role in placental development. Interestingly, we observed a higher predicted gestational age for placentas from early onset preeclampsia compared to their gestational age at delivery suggesting accelerated placental ageing. However, placentas from late onset preeclampsia showed no such ageing. Our data show that gestational age acceleration prediction from DNA methylation array data may offer an important insight into the molecular mechanisms of pregnancy disorders.