I am pregnant mum. I travel lots. When should I stop flying oversea?

It is not advisable to fly during first trimester as mums may feel high level of nausea ,exhaustion and sickness. And the risk of bleeding or miscarriage is quite high. Once you reach third trimester, the burden of carrying baby increases. And most airlines require a clearance letter signed by your gynae. So, I feel second trimester is the best time to fly.