16-18 hours after a sperm penetrated the egg cell, features characteristic of the fertilization of ovum can be observed under the microscope. A properly fertilized egg cell has two pronuclei – one comes from the egg cell and the other from the sperm, and two polar bodies, originating from the division of the egg cell’s genetic material.
Sometimes, fertilization anomalies can be observed. The presence of three, four or more pronuclei may indicate that more than one sperm penetrated the ovum, or that the ovum itself can have an abnormal amount of genetic material.