Whereas 2-3 follicles are acceptable in an IUI cycle, the same cannot be said for IVF cycle. In the context of IVF cycle too little understandably would not lead to a good pregnancy rate - the aim of fertility treatment.
The fact that too much can also be a problem is less commonly realized by patients. When the ovarian stimulation is being done in conjunction with IUI, a higher number of follicles that start growing could ultimately lead to a higher order multiple pregnancy with the set of complications that come with it. In IVF, if we think logically, more follicles should mean more eggs, more embryos and more success rate. However, where there are more than 15-20 eggs, a disproportionate number of eggs are immature and these do not contribute to the formation of embryos. More eggs does not therefore mean more good eggs and more good embryos.
To make matters worse is a complication called OHSS - Ovarian Hyper Stimulation Syndrome. OHSS happens as a result of a cascade of chemical reactions in the blood stream that results in leaky blood vessels. As a result fluid accumulates in the tummy cavity and sometimes in the chest around lungs and heart. This can be quite uncomfortable or even intensely painful to the woman. The leaking of fluid leads to the blood becoming thicker. Affected women are therefore at a risk of developing blood clots in the major veins. Deep vein thrombosis (as this condition is called), can occasionally cause death. Understandably, every IVF clinician strives to avoid this complication.