When is it unsafe to fly?

Ry
The first week of May I have a trip planned to Washington, D.C. (From where I live in Washington state) to go with my mom who participates in a study on NF2, the brain tumor disease she has. I don't HAVE to go, but next year is her last year and I'll have a baby to take care of so I'd much rather go this year if it's safer. I'll be 29 weeks when we leave and 30 when we come back. I don't have another appointment with my doctor for a few weeks and I really wanna know what you guys think.. Is it unsafe to go? And could someone please explain to me the dangers of flying while pregnant because I don't understand. Thank you!