Cardio exercise throughout pregnancy

I need to do 20-40 mins of cardio a day to hopefully achieve my goal of not taking an Antidepressant until my last trimester of pregnancy. There is no proof that antidepressants affect the baby long after birth but any medication that alters the brain chemicals freaks me out.

I have severe depression and I got off all my meds (with the doctor's help) as soon as I found out I was pregnant. My OBGYN suggested 20-40 mins of cardio exercise a day. I thought about an exercise bike because once I'm farther along and by belly is huge I can still do cardio without putting a bunch of pressure on my belly. Any other opinions would be nice. I'm an asthmatic that can't take allergy meds to help maintain it so that's a factor too