Santiago Brugo Olmedo M.D.
Medical Director
Seremas Medicine for men and women
Seremas Home Seremas Procedures Seremas High Complexity Assisted Reproduction Techniques Seremas IVF


Fertilization is a process which starts with the interaction between spermatozoa and the cover surrounding the oocyte (zona pellucida) and finishes with the disappearance of pronuclei in a process called syngamy.

In IVF, in order to make fertilization happen, each oocyte is incubated in the same culture medium together with approximately 50.000 to 100.000 spermatozoa previously prepared at the laboratory. At the moment when a sperm succeeds in going through the zona pellucida, the oocyte reacts activating this cellular barrier in order to block the entrance of other spermatozoa.

The evidence of fertilization is given by the visualization under microscope of the pronuclei (male and female), 16-20 hours after the co-incubation of both gametes.

Mature Oocyte
Spermatozoa examined under the microscope

One of the risks of IVF is fertilization failure. This happens in 1-3% of the cases. In case none of the oocytes is fertilized, or the embryos stop developing, the physician will not perform the embryo transfer and the procedure will be cancelled.

If gametes are normal, fertilization rate is approximately 70-80%. This rate varies depending on the gametes morphological characteristics, the woman’s age and cause of infertility. Other facts have influence on fertilization rate as well, among them, environmental variables such as quality and indemnity of culture media, purity of air and physical environment inside the incubators.

Normally Fertilized Oocyte with two pronuclei

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