Isolated teratozoospermia is defined as having less than 10% of normal forms, based on Kruger's strict criteria. A study published in 2003 showed that this type of teratozoospermia results in a lower cumulative birth rate compared to men with normozoospermia. (Spiessens C1, Vanderschueren D, Meuleman C, D'Hooghe T. Isolated teratozoospermia and intrauterine insemination, Fertil Steril. 2003 Nov;80(5):1185-9.)