I used heartgaurd and I can't say for sure this is to blame BUT I gave it to my bred (just bred) Chi not even thinking and ended up with a cleft palate pup. I now will not use ANYTHING on a female who I'm even just "thinking" about breeding.
My cleft palate baby made it, it was a hard long road for me, (and she is still with me and love her to death!!!!!
) but I will never chance it again.