The reduced number of semen production is one of the predominant causes of infertility in males. Semen may contain few spermatozoa (oligozoospermia) or no spermatozoa (azoospermia) due to problems in its production in the testicles. The prominent causes for low semen production are due to:
Disorder of the brain glands (Hypophysis and Hypothalamus that generate the hormones regulating the formation of spermatozoa), Genetic disorders (Klinefelter syndrome, alteration of genes contained in the Y-chromosome that regulates the formation of spermatozoa, other numerous testicular problems, such as underdevelopment, anomalous development, failure to descend into the scrotum (cryptorchidism), trauma, and inflammations (whether sexually transmitted or otherwise), tumors, exposure to toxic substances, chemotherapy and radiotherapy and varicocele.
Genital tract disorders stop semen from being deposited in the vagina during intercourse (erectile impotence, absence of ejaculation). In addition, there could be an abnormal situation of the urinary meatus like
If it is below its normal location (hypospadias) or above it (epispadias)
If the penis has a curvature that is too pronounced or a decrease in its size
Other general problems are Obesity and other erection problems, premature or delayed ejaculation and other ejaculation problems that can also prevent the spermatozoa from being adequately deposited in the vagina.