its due to the wire obstructing growth and the milk glands while they grow during pregnancy also because they dont support ytou as well as a maternity bra or nursing bra.
thats the medical reason! my reason is because when you start to get big the wire under your boobs really flippin digs in and hurts like hell! especially when your sitting down!
one thing tho buy a really good fit maternity bra and go for a more expensive one.
i found it so uncomfortable to wear an underwire bra again after my kids cos you wear a non wire for soo long and get really used to it. im in a habit now of changing into a non wire bra when i get home in the evenings!