it depends on how tight you are talking about. But by and by it is not UNSAFE to wear tight fitting clothes during pregnancy . Before there were Obstetricians' midwife's bound pregnant women's clothes. A form of a girdle to help support the lower back and upper shoulder girdle. But as in everything ..EXTREMES are to be avoided.

It is not advisable to wear tight clothing. It's comfortable to wear mainly cotton, loose cloth to help to cool down your skin surface temperature.
Good luck.

Anjan Chaudhury, MD, FACOG