Second trimester headaches - what gives??

This is my second pregnancy, and just like the first time, I’m at week 16 and have a throbbing headache nearly all day every day with NO explanation. They lasted until about week 22 last time.

I know the typical remedies. Tylenol, caffeine, ice, etc. but what I want to know is WHY does this happen to us? It seems fairly common, I’ve heard lots of other have the same experience.

Is something happening with the baby/placenta at this time that could cause headaches? Does anyone have any theories?