Its ok just have to watch it is all. With pregnancy your hormones change and can cause your hair to change too...
The texture (go from curly or to wavy or to bone limp straight) the density can change (get super thick or lose it and it thins)
it also may fry easier due to dryness!
Why wouldnt u be able to straighten ur hair? Haha some docs say u can even dye it
Lol Ive never heard anything wrong with str8ing your hair while pregnant ... Your fine !