Zygote at the two pronuclei stage, visible side by side in the centre of the oocyte. The 2 polar bodies are visible at 12h.

Once a spermEjaculated spermatozoa and secretions during the male orgasm. has penetrated the oocyteA female reproductive cell. At birth, the ovaries contain 1 to 2 million oocytes. At puberty, there are no more than 400'000 to 500'000 left. Only 400 to 500 will reach maturity and ovulation at a rate of  one per month until menopause. All the others will finally degenerate at various stages of development., the genetic material from the father and mother appear in the form of structures called pronuclei.

So, 16-20 hours post-fertilizationFusion of an egg and a spermatozoon, fertilization occurs naturally in the fallopian tubes., the zygotes (fertilized eggs) are identified by the presence of two pronuclei (maternal and paternal nuclei) and two polar bodies.

In general, two zygotes are cultured for the transfer of fresh embryos.

in accordance with the Swiss law (LPMAThe Swiss Law on Medically Assisted Procreation (2001).) , all supernumerary zygotes are frozen immediately (this must be done before the 20th hour, corresponding to when the pronuclei disappear and merge their content).