Hello, could you tell me if I can fly to another city? The due date is…

Hello, could you tell me if I can fly to another city? The due date is 21 weeks, I'll be back around week 23-24. Do I need a doctor's certificate for Kazakhstan? Everything is fine with my ultrasound and blood tests, but my gynecologist, who I am attached to, strictly forbids me from leaving. It doesn't matter if there's a risk or not, probably she doesn't want any responsibility. Has anyone flown before? What would you recommend? Or should I take a train? But I think it's better to fly for two hours than to travel by train for two days. I have been able to handle flights during pregnancy.