Short term disability for maternity leave

Help!! I enrolled in short term disability because my fiancé and I are ttc. The effective date starts March 1st and the insurance lady told me there is no waiting period I just can’t get pregnant prior to March 1st otherwise I won’t get to use my benefits. I just got my period today, so 2 days before it kicks in… if I get pregnant in March will they consider the pregnancy pre-existing since doctors use the first day of your last period to determine due date???? Or will they just go based on when I first receive prenatal care and have the pregnancy confirmed? I don’t want to get screwed out of my benefits but I’m sooo close I don’t want to have to wait another month and a half to start trying.