analysis with multi dependent factors

How to analyze the blood pressure decreased by a treatment, but the baseline was different

let say I have group x and group y

the group x (with no treatment) baseline blood pressure was 120
at the follow-up it was 120

group y baseline blood pressure was 160
at the follow-up it was 120

So group y had great decrease in blood pressure, but both groups were pregnant women the baseline was measured in 1st trimester while the follow-up was at the end of pregnancy what if the treatment had no effect and either way the blood would be decreased to 120 for all women ?!