As long as you have no other complications during your pregnancy then it's considered safe to fly at all times during your pregnancy.
So in most cases flying during pregnancy during your first trimester is generally considered to be safe but if possible it's best to avoid flying because in some cases the flying during your first trimester can indeed result in a miscarriage.
It's usually best to wait until the pregnancy is over with and the baby is born before flying or at least wait until further into the pregnancy.
Anything can happen though so I would not recommend flying during the first trimester just to prevent the chance of a miscarriage because it can and does happen although it's considered to be safe.