Any ideas as to what is causing my migraines?

I’m a 34yo woman, mother to three children. I got my first migraine at 15 but it wasn’t until I got into my 20’s that I started tracking them. My migraines always start with a visual aura - a spot of light that expands into a ring and then eventually disappears once it gets to the edges of my vision. I then have 20-30mins to get home before the pain starts. The pain stays behind my eyes, behind the browbone or just above. There is tingling down my arms into my hands and my feet. I am always nauseous and vomitting helps alleviate the pain. If I am able to fall asleep during or after the aura, I can skip the worst of the pain and wake up with a tender head, but functioning. The whole episode lasts 4-5 hours. Which brings me to my question. I’ve written down every date and time of every migraine for the last decade. They occur once every 4 months, almost to the week. They stopped when I got pregnant, except for my third child. I got two migraines 2 weeks apart at 8 weeks pregnant, but otherwise my pregnancies were all migraine-free. They returned about a year after birth and resumed the 4 month frequency.

These days they are occurring less frequently, much to my relief. I’ve always thought mine must be hormone related but I’m curious if anyone here has ideas as to what’s been happening and why they’re becoming less frequent? I’m new here and learning a lot! Thanks.