Mums who were diagnosed with high blood pressure before pregnancy, will have better outcomes if their blood pressure is easily controlled early in the pregnancy. It is possible that mums who adapt better early in pregnancy, are more fit than those who do not.
Reference: Nzelu D, Dumitrascu-Biris D, Nicolaides KH, Kametas NA. Chronic hypertension: first-trimester blood pressure control and likelihood of severe hypertension, preeclampsia, and small for gestational age. American Journal of Obstetrics and Gynecology. 2018 Mar 1;218(3):337-e1.