Zygote

A zygote is the first stage of embryo development, formed when a sperm cell successfully fertilizes a mature oocyte. At this point, two nuclei — one from each gamete — are visible under the microscope as pronuclei (2PN), confirming normal fertilization. This stage lasts roughly 16–24 hours after insemination or ICSI.

The zygote then undergoes cleavage divisions to become a 2-cell, 4-cell, and 8-cell embryo over the next few days, eventually developing into a morula and then a blastocyst by day 5 or 6.

ZIFT (zygote intrafallopian transfer) is a now-rarely-used procedure in which the zygote is placed into the fallopian tube rather than the uterus.