Analysis of fetal weight between 4 time periods

I have a study in which 283 fetuses have had their weight measured at 4 different time periods. Period I needs to be compared to Period 2. Period 2 compared to Period 3, and period 2 compared to period 4. I need to see the graphical display of the results and the P values. The data is nonparametric and the analysis needs to be done using non-parametric smoothing function of restricted cubic splines