Multiple pregnancies and ovarian hyper stimulation syndrome (OHSS) are the most serious complications of ovarian stimulation and IVF. Indeed, they are more or less the only ones.OHSS can effectively be prevented in three stages: Before stimulation, during stimulation and at ET.Before stimulation patients should be assessed as to age, the ovarian follicular count and/or anti mullerian hormone (AMH). A correct starting dose of FSH should hence be administered.During stimulation and ET many different strategies can be applied: 1. Low dose stimulation: The disadvantage is fewer oocytes and fewer embryos to freeze.2. GnRH antagonist protocol with oocyte maturation by administering a GnRH analogue instead of HCG. Disadvantage is the handling of the luteal phase after ET in the fresh cycle.3.Coasting before HCG is administered. Disadvantage is that it does not always work and requires a lot of experience.4. No fresh ET, freezing of all appropriate embryos. Disadvantage is that it does not influence the first phase OHSS.5.Dopamin agonist prescribed before HCG is administered.Disadvantage is that it does not affect OHSS if pregnancy occurs.6.Transferring no more than one embryo. Disadvantage, if any, is that it may decrease the pregnancy rate.7.Cancelling the cycle.These tools make it perfectly possible to avoid any serious OHSS.Given a good freezing programme, the disadvantages are minor. Serious OHSS should no longer be a part of IVF.