Fertilization and pre-implantation phase

Fertilization (stage 1)

Human life begins with fertilization. This is a process that starts with the penetration of a spermatozoon (sperm) into the secondary oocyte followed by the union of maternal and paternal chromosomes. Fertilization is complete when the zygote begins to divide, after about 24 hours. Normally there is only one day between ovulation and fertilization. Therefore, the age of the fetus taking ovulation as the starting point (postovulatory age) can be considered the same as when counting from fertilization (postfertilization age).
Shortly before ovulation, a tertiary follicle matures to become the Graafian follicle (oogenesis). The oocyte is surrounded by the zona pellucida and the corona radiata (a crown of granulosa cells). The primary oocyte terminates its first maturation division (meiosis), with chromatin being distributed equally to each of the two daughter cells. One of the daughter cells has very little cytoplasm and forms the first polar body, which eventually degenerates. The cytoplasm-rich daughter cell forms the secondary oocyte. Both cells remain in the zona pellucida. The enlargement of the Graafian follicle causes the surface of the ovary to protrude and an ischemic white spot (stigma) is formed where the follicle is at its widest. Ovulation occurs when the follicle ruptures and the oocyte is released. The oocyte (ovum) is then captured by the fimbriae and transported along the uterine tube. During ovulation, the oocyte starts its second maturation division (meiosis II), which is continued up until the metaphase. Fertilization normally occurs in the ampulla of the uterine tube. The oocyte is receptive to fertilization for around 24 hours; if it does not happen during this timeframe then the oocyte dies.