The aim of this report is to create a statistical model that can help answer the question whether twins tend to suffer from placental dysfunction more often than singletons or if twins by some genetic conditions have smaller normal weight than singletons. To answer this question, data from Aalborg University Hospital was used. The data was of hierarchical nature and composed of two levels - one for each individual mother and one for each individual fetus. In order to incorporate this nature mixed effects models was used. During this report theory about mixed effects models will be presented. Multiple models will be fitted and tested to make sure all model assumptions are met. Among seven models the model with lowest Akaike information criterion (AIC) and Bayesian information criterion (BIC) will be chosen as the one that models the data best. Based on this model it is seen that the estimated fetal weigth for twins at a given placental T2* value is significantly different from singletons which could indicate that twins have a smaller normal weight than singletons. Hence, there might be a need for new reference curves when assessing the size of twin fetuses and thereby the placenta function.