Therefore, our key findings are that (1) there are minimal effects of menstrual phase or OC on hemodynamic or cerebrovascular resistance responses to hypercapnia, (2) all women exhibit enhanced HF and VLF dCA via dampening of blood pressure fluctuations (nGain) in hypercapnia, and (3) while naturally cycling women have reduced timing (Phase) of LF changes in blood pressure translating to changes in CBF during hypercapnia in the HH menstrual phase, OC users exhibit the same reduction in the LH menstrual phase. The gene discussed is PLOD1; the disease is hydrops fetalis.