In a study conducted by the VU University Medical Centre in Amsterdam in the Netherlands, has indicated that women that give birth to multiple babies following IVF have a slightly higher risk of breast cancer when compared to those who give birth to single babies or do not have any children. In a study which included a follow up of 1688 women, the study showed that women that delivered multiple pregnancies, including twins and triplets, seemed to have a slightly higher risk of breast cancer when compared to women who had single babies or who had not fallen pregnant.
It is not entirely certain what would be the cause, but it is presumed that the high levels of oestrogen and progesterone that accompany multiple pregnancies may encourage some excessive self-division which leads to breast cancer. It could also be a possibility of a protein that is increased in pregnancy. These are very early studies and have not yet become a recommendation