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and eight-cell stages. Within seventy-two hours, the zygote develops into the morula, the solid mass of blastomeres formed by the cleavage of the fertilized ovum (egg). After about three to five days in culture, the zygote has become a hollow ball of cells called the blastocyst. During normal embryogenesis, it is the blastocyst that is implanted in the endometrial lining of the uterus.

While the embryos are in culture, problems in their development may become apparent. After embryos with evident problems are discarded, one or more cultured embryos are transferred into the uterus, where, it is hoped, one will become implanted and develop into a healthy, full-term baby. Embryo transfer replaces the natural process in which the embryo passes down the fallopian tube and into the uterus, prior to implantation.

Although this transfer is relatively simple and often takes only a few minutes, the rate of successful implantation is low. Usually, two embryos are transferred, but still only one of five women become pregnant. Most doctors who perform IVF procedures adhere to the limit of two embryos per woman to minimize the risk of multiple pregnancies. In most cases, three embryos are transferred in women older than thirty-five. In the United Kingdom, it is illegal for an IVF doctor to transfer more than three embryos at a time into a woman.

A number of factors play a role in whether embryo transfer leads to a baby being born. Success rate is higher if embryo transfer takes place between forty-eight and seventy-two hours after oocyte collection. When more than one embryo is transferred at the same time, the success rate increases, but so does the chance for multiple pregnancies. As noted above, the maximum number transferred should never exceed three. Probably the single most important factor determining whether or not a successful embryo implantation will take place is the donated egg's age. Embryos formed from eggs donated by younger women have a higher implantation success rate than do embryos formed from eggs donated by older women. The age of the host uterus appears to have little or no effect on outcome.

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