Why do fertility treatments often cause multiple births?

Egg numbers: Fortunately, most higher order multiple pregnancies are mostly preventable. The more eggs and embryos, the higher the chance of multiple pregnancy. Proper fertility treatments can greatly reduce the risk of multiples, for instance conservative ovulation induction, and elective single embryo transfer (eset) after ivf.

Multiple oocytes: Because most fertility treatments involve stimulating multiple eggs (= targets for sperm.) there ar other reasons but this is the easiest to understand.