Frozen embryos were used in approximately 14% of all ART cycles performed in 2003 (17,517 cycles). Figure 36 compares the success rates for frozen embryos with the success rates for fresh embryos among women using their own eggs. Because some embryos do not survive the thawing process, the live birth per thaw rate is usually lower than the live birth per transfer rate. In 2003, the success rates for frozen embryos were lower than the success rates for fresh embryos. However, the average number of embryos transferred was similar for cycles using both frozen embryos and fresh embryos (see the National Summary Table for information on the average number of embryos transferred for these cycles). It is important to note that cycles using frozen embryos are both less expensive and less invasive than those using fresh embryos because the woman does not have to go through the fertility drug stimulation and egg retrieval steps again.

Multiple-infant births are associated with greater problems for both mothers and infants, including higher rates of caesarean section, prematurity, low birth weight, and infant disability or death.

Part A of Figure 37 shows that among the 5,381 pregnancies that resulted from ART cycles using frozen nondonor embryos, slightly more than 66% were singleton pregnancies, about 21% were twins, and nearly 4% were triplets or more. Nine percent of pregnancies ended in miscarriage before the number of fetuses could be accurately determined. Therefore, the percentage of pregnancies with more than one fetus might have been higher than what was reported (nearly 25%).

In 2003, 4,246 pregnancies from ART cycles that used frozen nondonor embryos resulted in live births. Part B of Figure 37 shows that approximately 25% of these live births produced more than one infant (about 23% twins and 2% triplets or more). This compares with a multiple-infant birth rate of slightly more than 3% in the general U.S. population.

Although the total rates for multiples were similar for pregnancies and live births, there were more triplet pregnancies than triplet births. Triplet (or more) pregnancies may be reduced to twins or singletons by the time of birth. This can happen naturally (e.g., fetal death), or a woman and her doctor may decide to reduce the number of fetuses using a procedure called multifetal pregnancy reduction. Information on medical multifetal pregnancy reductions is incomplete and therefore is not provided here.