When my daughter was in daycare we got pregnant with our second child after she had been in daycare for only 2 weeks. I didn't tell my provider right away because we just wanted to wait before we told everyone, however, my tummy had other ideas and it was obvious right from the start (seriously like at 4 weeks I had a little paunch). Anyways, my provider just came out and asked me, and while I embarrassed I just told her the truth, and she had no problems with it.
I was afraid to tell her too because I felt bad knowing I couldn't afford to keep my daughter in her care while on mat leave and knowing my daughter would only be there a short while, I felt like I was wasting her time.