Some clinics see more than the average number of patients with difficult infertility problems. Others discourage such patients or encourage them to use donor eggs, which have higher success rates among women older than 35. Clinics that accept a higher percentage of women with multiple previous unsuccessful attempts will generally have lower success rates than clinics that do not. Alternatively, some clinics may raise their success rates by offering ART procedures to patients who might have become pregnant with less technologically advanced treatment. In an unstimulated cycle, the woman ovulates naturally rather than through the daily injections required by stimulated cycles.
Unstimulated cycles are less expensive because they eliminate the cost of the injectable drugs.
They further reduce the cost of the procedure by reducing the number of ultrasounds and blood tests required. However, women who use natural or mild stimulation produce only one or two follicles, thus reducing the potential number of embryos for transfer. As a result, unstimulated cycles have lower success rates, and clinics that carry out a relatively high proportion of unstimulated cycles will have lower success rates than those that do not. Success rates for GIFT and ZIFT are reported together with those for IVF . Because success rates for GIFT and ZIFT are higher than rates for IVF, clinics that do more GIFT and ZIFT procedures will have higher success rates.