i haven't bought any 孕婦內褲 but i have changed one size larger in later gestation period.
i think ordinary underpants won't cause any harm to the bb's development but some may wear it as long as she feels it comfortable.
don't believe the sales saying that mammy's 肚臍係bb's臍帶位. it is not true, bb's臍帶 is linking to the placenta inside the uterus, not related to our 肚臍. don't worry!