The presence of a plug does not necessarily indicate the deposition of normal sperm within the female tract. It also must be noted that plug-negative females may, in fact, become pregnant. Pregnancy of females carrying very small litters may not be visually apparent. After the compaction phase of development, during the morula stage on ED 2.5, two factors affect the ultimate yield of blastocyst embryos. First, the predictable synchrony of mammalian preimplantation development is lost.
Different embryos exhibit variable rates of compaction and blastocoele development.
Therefore, ED 3.5 collections characteristically exhibit a wide variation of embryo devel opment. ES cell chimera production has been achieved with each of these embryonic stages, but the vast majority of protocols require the use of fully expanded blastocyst stage embryos as ES cell recipients. Second, the total number of viable blastocysts frequently decreases, relative to the number of ED 0.5 2.5 embryos. This attrition is coincident with the passage of the embryos from the oviduct into the uterus. The direct causes of this phenomenon are not well understood.