I have 3 groups of data. SGA: <5% birth weight, AGA: 5%≤birthweight≤95%, and LGA: >95% birth weight.

50 SGA and LGA cases were selected from a cohort, and matched to 50 AGA cases according to ethnicity. Hormones levels were measured.

I can't find any significant associations within each group. Can I run a multiple regression with these 150 cases altogether with hormones as independent variables to predict birth weight? How can I test it is reasonable to do that?

Many thanks.