Short term disability??

C

So my husband and I are ttc and I asked my work what I should do to prepare and they told me to get short term disability. So I did but they said I cant already be pregnant because that is considered a pre existing condition. It kicks in october 1st and my husband and I were literally trying this week because I'm fertile and now I cant because if we get pregnant then it would be a pre existing condition!! How do they prove I was pregnant before the policy starts Oct 1???