Of all infertility treatment available, IVF-ET generally offers the highest chance of success per cycle. In 2008, 34 percent of all cycles resulted in a live birth. This rate has been improving slowly but steadily over the years. For example, the corresponding national rate for 1989 was only 14 percent. The delivery rate or "take home baby rate" is the only real measure of success. Patients should be aware, however, that some clinics define "success" as any positive pregnancy test or any pregnancy, even if miscarried or ectopic. These "successes" are irrelevant to patients desiring a baby. To put these figures into perspective, studies have shown that the rate of pregnancy in couples with proven fertility in the past is only about 20 percent per cycle. Therefore, although a figure of 34 percent may sound low, it is greater than the chance that a fertile couple will conceive in any given cycle.
Success varies with many factors. The age of the woman is the most important factor, when women are using their own eggs. Success rates decline as women age, and success rates drop off even more dramatically after about age 37. Part of this decline is due to a lower chance of getting pregnant from ART, and part is due to a higher risk of miscarriage with increasing age, especially over age 40. There is, however, no evidence that the risk of birth defects or chromosome abnormalities (such as Down syndrome) is any different with ART than with natural conception.
Success rates vary with the number of embryos transferred. However, transferring more and more embryos at one time does not continue to increase the chance of success, but may only increase the risk of a multiple pregnancy, which is much more complicated than a singleton pregnancy and is more likely to result in babies with severe medical problems. The impact of the number of embryos that are transferred on success rates also varies with the age of the woman. For example, for many women under 35 years of age, transferring more than two embryos does not increase the chance to get pregnant, but only increases the risk of multiple pregnancy.
The American Society for Reproductive Medicine (ASRM) has published guidelines for the recommended number of embryos to transfer. Presently, the collection of oocytes, fertilization, and early embryo growth are accomplished with a high degree of efficiency. The major hurdles to success are implantation after embryo transfer and early pregnancy loss. We still do not understand all the factors that influence the potential of embryos to implant and produce successful pregnancies. The rate of early pregnancy loss is slightly, but not significantly, higher with ART compared to spontaneous conception.
Pregnancy complications, such as premature birth and low birth weight, tend to be higher with ART pregnancies, primarily because of the much higher rate of multiple pregnancy. About 30 percent of ART deliveries are twin deliveries, versus 1-2 percent of spontaneous pregnancies. The risk of more than a twin delivery (i.e., triplets or more) is less than 5 percent.
Couples may require several cycles of treatment to have a baby. Success rates remain fairly constant over several cycles, but may vary greatly between individuals. Couples who have achieved an ART pregnancy and delivery in the past have a slightly increased likelihood of success. Couples who have frozen embryos may attempt to conceive using these embryos. Generally, these cycles are less complicated and require fewer visits and injections that fresh IVF cycles. Sometimes, an interval of at least one menstrual cycle is recommended between fresh IVF cycles.