In a small number of women, normal pregnancy is not possible because of some anatomical or physiological condition. In 87% of these cases the uterine tubes are incapable of transporting the zygote to the uterus or of allowing sperm cells to reach the oocyte. In vitro fertilization and embryo transfer have made pregnancy possible in hundreds of such women since 1978. In vitro fertilization (IVF) involves removal of secondary oocytes from a woman, placing the oocytes into a petri dish, and adding sperm cells to the dish, allowing fertilization and early development to occur in vitro, which means "in glass". Embryo transfer involves the removal of the developing embryonic cellular mass (not actually an embryo) from the petri dish and introduction of the mass into the uterus of a recipient female.

For IVF and embryo transfer to be accomplished, a woman is first injected with an LH-like substance, which causes more than one follicle to ovulate at one time. Just before the follicles rupture, the secondary oocytes are surgically removed from the ovary. The oocytes are then incubated in a dish and maintained at body temperature for 6 hours. Then sperm cells are added to the dish.

After 24 to 48 hours, when the zygotes have divided to form two-to eight-cell masses, several of the embryonic masses are transferred to the uterus. Several cell masses are transferred, because only a small percent of them survives. Implantation and subsequent development then proceed in the uterus as they would for natural implantation. However, the woman is usually required to lie perfectly still for several hours after the cell masses have been introduced into the uterus to prevent possible expulsion before implantation can occur. It is not fully understood why such expulsion does not occur in natural fertilization and implantation.

The success rate of embryo transfer is actually quite low. As of May 1993, the overall birth rate from 16,405 oocyte retrievals in all clinics was only 14 %, although some clinics report twice that success level. The success rate varies with the number of embryonic masses implanted per transfer: 13% for transfers of three or less embryonic masses, 25% with four embryonic masses, and 26% with five or six embryonic masses. However the rate of complications, such as multiple pregnancies, miscarriage, and prematurity also increases with increased numbers of embryonic masses per transfer. Of triplets born as a result of IVF, 64% required intensive care after birth, and 75% of quadruplets required intensive care, often for several weeks. Prematurity from IVF pregnancies in the United Kingdom resulted in newborn mortality in 2.7% in cases, a rate three times that of natural pregnancies. As a result of these complications, no more than two to three embryonic masses are now transferred per IVF in the United Kingdom.

The success rate also varies from clinic to clinic but its increasing steadily. The success rate at the best U.S. clinic was 20% in 1982 and 27% in l989. One clinic in the United Kingdom now reports a success rate of 37% to 42% following transfers of only two to three embryonic masses, with a triplet rate of less than 1%. This success rate may be approaching the natural limits, because only 50% or less of natural fertilizations result in a successful delivery.

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