Alltough I do make a distinction (I'm always curious
), I agree with pjt33 that the term is very relaxed, and in an usual conversation you can hear both with no proper signification.
@ irmamar
"De todos modos, un médico te hablará de "embarazo gemelar", sean gemelos o mellizos (univitelino o bivitelino). "
Es correcto, pero yo creo que se cura en salud, el médico no sabe si son mellizos hasta tarde ...
Saludos