Semen consists of sperm and various other fluids that act as a vehicle and nutrients for the sperm. The volume, colour and consistency of the ejaculate remain the same after vasectomy. After the vasectomy it is not possible for sperm from the testicle to get through to the urethra because the cutting of the vas interrupts this passageway. However in the early weeks after the vasectomy there may still be some sperm present in the system of tubing beyond the severed portion of the vas. This is the reason why the contraceptive effects of vasectomy are not immediate and additional contraceptive precautions have to be taken for several months. The semen is analysed at 16 and 20 weeks post vasectomy in order to ensure that it is free of sperm. Even at this point the appearance and volume of ejaculate should be indistinguishable from the pre vasectomy ejaculate. The only difference is that there should not be any sperm present when the ejaculate is examined under the microscope. It is only at this point that additional contraceptive precautions can be dispensed with.