My DD was born 11 weeks prem, she was an ICSI bub. Prematurity was due to placenta previa resulting in placental abruption. I have since read that IVF has more risk of placenta previa due to transfers being done lower in the cervix as it is more likely to implant. Don't know if this is true or not.
I would also think that prematurity would be due to the higher incidence of mutiple birth.