If a man's sperm count is low (less than 20 million per millilitre), then this could definitely be reducing your chances of conceiving. Levels of 40 million would be much better.
However, sometimes the problem isn't so much low numbers of sperm, but high levels of abnormality. All men have a percentage of abnormal sperm, and up to 70 per cent is considered acceptable. The abnormal sperm can have two heads or no tails, for instance, but only if there is a very high percentage of abnormal sperm will a man's fertility be affected.Many specialists believe that these abnormal sperm would find it difficult to get to the fallopian tube and, once there, would not be able to penetrate an egg. But some studies have shown that abnormal sperm are in fact capable of reaching the fallopian tube (Proceedings of the INSERM International Symposium, Nouzilly, 1975: 63-73).
Others claim that a high concentration of abnormal sperm could be connected to a high rate of miscarriages (JAMA, 1951; 146: 1212-8).
One study, published in the Lancet in 1998, showed that a man with a low sperm count can still be fertile as long as there is a high proportion of normal sperm (Lancet, 1998; 352: 1172-7).