No-fly times?

KDee • Tried for almost 5 years, with every test and almost every fertility procedure, until I finally started telling the doctors which tests to do. Hoping this rainbow baby makes it and stays!
Does anyone know when the usual cutoff is for flying? My sis is having a baby in a month. I'd love to fly out and meet her. But I don't even know where to start. Do different airlines have different rules? When do doctors usually say no? And if I have a low-lying Placenta that isn't previa (yet) will That keep me earth-bound? I don't want to risk anything, but I'd love to go.