正常下限の精液数は不妊に関係あるかもしれない　

1998/10/9ロイター　精子濃度20万個/mlは正常とされているが、異常な形の精子の多い場合は不妊になることが多い。

Low normal sperm count may affect fertility

NEW YORK, Oct 09 (Reuters) -- Some men with sperm counts at the low end of what is considered ``normal'' -- 20 million sperm per milliliter of semen -- may have fertility problems, according to a Danish study published in the October 10th issue of The Lancet.

Not only sperm concentration, but sperm shape affects fertility, the findings suggest. Among men with low-normal sperm counts, those with a high proportion of normally-shaped sperm may have normal fertility, while those with a high proportion of abnormally-shaped sperm have impaired fertility, lead researcher Dr. Jens Peter E. Bonde, of University Hospital of Aarhus, in Denmark, told Reuters Health in an interview.

Bonde and colleagues studied a group of roughly 400 men, ages 20 to 35, who were planning a first pregnancy with their partner. The researchers followed the couples through six menstrual cycles, or until conception, whichever occurred first. The couples had unprotected sex at least once during the fertile phase of each cycle. During each cycle, the men collected one semen sample, which Bonde and colleagues analyzed, assessing sperm concentration, prevalence of normal-shaped sperm, and sperm motility.

Roughly 250, or 60% of the couples conceived during the 6-month study period, the researchers found. Sixty-five percent of men with sperm concentrations of 40 million per milliliter of semen or higher conceived, compared with 51% of men with lower sperm concentrations. Only 36% of men with sperm concentrations of less than 20 million per ml conceived, Bonde and colleagues report.

But sperm count was not the only thing that mattered, the researchers report.

``In men having a sperm count around 20 million per milliliter, men with a high proportion of normal-shaped sperm would have a higher probability of conception than men with a low proportion of normal sperm cells,'' Bonde told Reuters Health. ``However, sperm count and sperm shape is correlated: if the sperm count is low there is also a higher probability that the proportion of abnormal sperm is high.''

According to World Health Organization guidelines, the lower limit of the normal sperm concentration range is 20 million per milliliter of sperm.

``Our findings suggest that the current WHO guidelines for normal semen quality should be used with caution,'' the researchers conclude. ``Some men with sperm counts above the lower limit of the normal range defined by WHO (20 x 10 to the 6th power/ml) may in fact be subfertile.''

Although other research has found that sperm motility also affects the likelihood of conception, the researchers found that the proportion of non-motile sperm had ``little effect on likelihood of pregnancy.'' This may be because of error in their analysis, however, Bonde said.

SOURCE: The Lancet 1998;352:1172-1177.