All fertility treatment cycles involving ovarian stimulating medication have similar risks but the difference is the degree of severity. For cycles using only oral or injectable fertility medications, the risks of an ectopic pregnancy, ovarian hyperstimulation syndrome and ovarian torsion are less than 5% in both IUI and IVF cycles.
The difference between IUI and IVF are the pregnancy success rates and risk of multiple births. In an IUI cycle using oral medication, the risk of a multiple pregnancy is less than 5% but increase to 10-15% when injectable fertility medications are used.
The unique distinction of IVF is the ultrasound guided vaginal egg retrieval and the ability to transfer a predetermined number of embryos. As a result, the risks with an egg retrieval included intra-abdominal bleeding, injury to the bowel and bladder, and infection. Fortunately, all of complications occur in less than 1% of cycles. Further, the risk of a multiple pregnancy is directly related to the number of embryos transferred: a single embryo transfer has a 1% risk of dividing into a twin pregnancy; a double embryo transfer can increase the risk of a twin pregnancy in up to 40% of cycles.