During intercourse ejaculate is deposited around the cervix. Only progressively motile sperm having a normal morphology can penetrate and migrate through the cervical mucus.
In addition, there are factors in seminal plasma that actually inhibit fertilization (de capacitation factors), and prostaglandin. Although fertile sperm with progressive motility and normal morphology are able to pass through into the uterus, they can’t fertilize an egg until its seminal plasma coating are removed.
The sperm processing performed in the laboratory removes these harmful substances, along with dead or damaged sperm. The processing also concentrates all viable sperm into an amount ideally suited for Intra Uterine Insemination (IUI), a process by which a gynaclgist clinically place sperm within the uterus directly thrugh the cervix.