Most cases of male infertility are related to the quality or quantity of sperm in ejaculated semen. According to the most recent references from the World Health Organization, 15 million sperm per millilitre, or less than 40 million sperm total per ejaculate, is considered below the normal range.
There is some controversial evidence that sperm counts have been falling over the past decades - although the IVF technique of ICSI (intracytoplasmic sperm injection) needs just one sperm cell to achieve fertilisation.
Poor sperm quality might also hinder fertilisation, such that sperm cells might be insufficiently motile to swim along the reproductive tract to meet an egg released at ovulation. Some men might produce no sperm cells in the ejaculate, a condition known as azoospermia. Yet ICSI can help even these men by extracting just a few sperm cells from the testis for injection into the egg.
Male fertility is tested by semen analysis, which tests sperm count, sperm motility and shape. A normal sperm count would be measured at around 20 million sperm per millilitre of semen. There are many reasons for a below normal sperm count, which investigation would hope to explain.
While female fertility declines with age and ends with the menopause, men have no such obvious change in their fertility. However, recent studies do show that the genetic composition of sperm does deteriorate with age, and that success rates in IVF, where the male partner is over the age of 40, are slightly lower than with younger male partners. Most national sperm donation programmes will not allow donors over the age of 40 or 45.
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