Adverse trends in male reproductive health: we may have reached a crucial 'tipping point'

Abstract

Healthy men produce an enormous number of sperms, far more than necessary for conception. However, several studies suggest that semen samples where the concentration of sperms is below 40 mill/mL may be associated with longer time to pregnancy or even subfertility, and specimens where the concentration of sperms is below 15 mill/mL may carry a high risk of infertility. Historic data from the 1940s show that the bulk of young men at that time had sperm counts far above 40 mill/mL with averages higher than 100 mill/mL. However, recent surveillance studies of young men from the general populations of young men in Northern Europe show that semen quality is much poorer. In Denmark approximately 40 percent of the men have now sperm counts below 40 mill/mL. A simulation assuming that average sperm count had declined from 100 mill/mL in 'old times' to a current level close to 40 mill/mL indicated that the first decline in average sperm number of 20-40 mill/mL might not have had much effect on pregnancy rates, as the majority of men would still have had counts far above the threshold value. However, due to the assumed decline in semen quality, the sperm counts of the majority of 20 year old European men are now so low that we may be close to the crucial tipping point of 40 mill/mL spermatozoa. Consequently, we must face the possibility of more infertile couples and lower fertility rates in the future.

Figure 1

From Bonde et al. (1998) Lancet: 430 couples with no previous reproductive experience, aged 20–35 years participated in a study on association between semen quality and the probability of conception in a single menstrual cycle. The couples discontinued use of contraception, and were followed up for six menstrual cycles or until a pregnancy was verified within this period. Each man was asked to provide a semen sample at enrolment. Women kept a daily record of vaginal bleeding and sexual activity. The association between semen quality and likelihood of pregnancy was assessed by logistic regression, adjusted for sexual activity and female factors associated with low fertility. There were 256 (59.5%) pregnancies among the 430 couples: 165 (65.0%) among those with a sperm concentration of 40 mill/mL or more, and 84 (51.2%) among those with lower sperm concentrations. The probability of conception increased with increasing sperm concentration up to 40 mill/mL, but any higher sperm density was not associated with additional likelihood of pregnancy.

Figure 2

Data from a recent surveillance project on semen quality including 3517 young men from the general population in Copenhagen (Jørgensen et al., 2006). The project was carried out during the years 1996–2005. The bars show the percentage of men according to categories of sperm concentration. The median sperm count of the population was 46 mill/mL during this period. Note that 42% of men had sperm counts below 40 mill/mL and therefore may belong to an at risk group of subfertility (Bonde et al., 1998). 20% were even below the WHO demarcation line of for subfertility (20 mill/mL).

Figure 3

Simulation showing a gradual decrease from A to D in average sperm counts of a population from 100 mill/mL (Similar to data from studies of the 1940s) to 40 mill/mL (close to the current situation). For simplicity the simulation is based on the assumption that sperm counts of men in a population are normally distributed, although distributions of sperm counts, particularly in populations with relatively low sperm counts often show a left-skewing of the distribution curves. Note the two hatched areas between the vertical lines demarcating sperm counts of <15 mill/mL (assumed high risk of subfertility) and <40 mill/mL (assumed moderate risk of subfertility). Note that a substantial decrease in sperm count does not significantly affect fecundity until average sperm count is 60 mill/mL. As the simulation (due to lack of skewing of the curves) slightly underestimates the proportion of men with the lowest sperm counts, even more men in populations with averages of 40–60 mill/mL sperms should in reality be within the hatched areas.