Typically, bovine embryos are transferred into recipient females about day 7 after estrus or anticipated ovulation, when the embryo has reached the blastocyst stage of development. All the biological and technical causes for failure of a female to produce a blastocyst 7 d after natural or artificial insemination (AI) are avoided when a blastocyst-stage embryo is transferred into the female. It is reasonable to expect, therefore, that pregnancy success would be higher for embryo transfer (ET) recipients than for inseminated females. This expectation is not usually met unless the recipient is exposed to heat stress or is classified as a repeat-breeder female. Rather, pregnancy success is generally similar for ET and AI. The implication is that either one or more of the technical aspects of ET have not yet been optimized or that underlying female fertility that causes an embryo to die before day 7 also causes it to die later in pregnancy. Improvements in pregnancy success after ET will depend upon making a better embryo, improving uterine receptivity, and forging new tools for production and transfer of embryos. Key to accelerating progress in improving pregnancy rates will be the identification of phenotypes or phenomes that allow the prediction of embryo competence for survival and maternal capacity to support embryonic development.

中文翻译：

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牛胚胎移植的承诺尚未完全兑现——为什么怀孕率没有更高，我们能做些什么呢？

通常，牛胚胎在发情或预期排卵后约第 7 天被转移到受体雌性体内，此时胚胎已达到囊胚发育阶段。当囊胚期胚胎移植到雌性体内时，可以避免自然或人工授精 ( AI ) 后 7 天雌性未能产生囊胚的所有生物学和技术原因。因此，可以合理地预期，胚胎移植 ( ET ) 受者的妊娠成功率将高于受精女性。除非受体暴露于热应激或被归类为重复繁殖雌性，否则这种期望通常不会得到满足。相反，ET 和 AI 的妊娠成功率通常相似。这意味着 ET 的一个或多个技术方面尚未优化，或者导致胚胎在第 7 天之前死亡的潜在女性生育能力也会导致其在怀孕后期死亡。ET 后妊娠成功率的提高将取决于制造更好的胚胎、改善子宫容受性以及打造用于胚胎生产和移植的新工具。加速提高妊娠率的关键是识别表型或表型，从而预测胚胎的生存能力和母体支持胚胎发育的能力。