He won't let me go

I'm in my second trimester with twins (16 weeks) and as long as I'm drinking the amount of water im supposed to, I use the restroom pretty frequently. I wouldn't say it's "too much", but my boss (a man, duh) is throwing hissy fits about it. In an 8 hour shift I maybe use the restroom a couple times an hour, 3 at the very most. I am quick in the bathroom and return to my post swiftly- I usually keep time on my phone to be sure it's never more than 3-4 minutes (because I myself think 5 minutes is excessive.) Last week. My boss told me that whenever I leave the front desk, I need to tell him. I was furious! My other coworkers leave as many as 11 or 12 times a day to smoke cigarettes, others go sit in the back room for a few minutes to text or play with their iPads, and I can't go pee?!? He told me to avoid having to tell him EVERY time I go, I would have to get a doctors note. I did, but reluctantly. Should I have had to go as far as to get a doctor to sign off on me having increased restroom breaks while pregnant? I don't feel it's taking away from my job, it's not as if I spend more one peeing than I do helping clients! We're his actions right, or wrong? 

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