Effects of ejaculatory frequency and season on variations in semen quality  Elisabeth Carlsen, M.D., Jørgen Holm Petersen, Ph.D., Anna-Maria Andersson, Ph.D., Niels E. Skakkebaek, M.D.  Fertility and Sterility  Volume 82, Issue 2, Pages 358-366 (August 2004) DOI: 10.1016/j.fertnstert.2004.01.039

FIGURE 1 Estimated effect of an individual's duration of abstinence on sperm concentration, shown both as a smoothed curve and as a piece-wise linear function (bold) (<4 days, 4–6 days, and >6 days). Note that increasing abstinence time is associated with increasing sperm concentration. Left-hand vertical axis shows each individual's sperm concentration for different durations of abstinence relative to his sperm concentration with an abstinence period of 2.5 days. From the right-hand vertical axis an individual's sperm concentration can be estimated at different abstinence times, assuming that his sperm concentration is 50 × 106/mL when the abstinence period is 2.5 days. Carlsen. Longitudinal semen variation. Fertil Steril 2004. Fertility and Sterility 2004 82, 358-366DOI: (10.1016/j.fertnstert.2004.01.039)

FIGURE 2 Estimated effect of season, relative to January 1, on sperm concentration. Vertical axis shows each individual's sperm concentration at different times during the study period relative to his concentration on January 1. The smoothed curve and the sinoidal curve (bold), which are in fairly good agreement, show that sperm concentration tends to be higher in May compared with October. The difference, which from the figure is seen to be approximately 15%, is not statistically significant (sinoidal, df = 2, P=.49). Carlsen. Longitudinal semen variation. Fertil Steril 2004. Fertility and Sterility 2004 82, 358-366DOI: (10.1016/j.fertnstert.2004.01.039)

FIGURE 3 Annual changes in the average number of ejaculations per day (rate). The rate is significantly higher during spring (months 3–5) compared with winter (months 12–14) (P=.0012). However a smaller, insignificant increment is also seen in August and October (months 8, 10). Carlsen. Longitudinal semen variation. Fertil Steril 2004. Fertility and Sterility 2004 82, 358-366DOI: (10.1016/j.fertnstert.2004.01.039)

FIGURE 4 (A) The 7-day observation period in relation to the actual semen sampling. The arrow with ejaculation indicates the last ejaculation before semen sampling. The abstinence time is the actual abstinence time, which varies in each case. (B) Histogram illustrating the distribution of numbers of ejaculations during the 7-day observation period before the abstinence period for all individuals. It seems that 3–5 ejaculations are the most frequent numbers of ejaculations (median 5) during this 7-day period. Carlsen. Longitudinal semen variation. Fertil Steril 2004. Fertility and Sterility 2004 82, 358-366DOI: (10.1016/j.fertnstert.2004.01.039)

FIGURE 5 The intraindividual coefficients of variations (CV) (percent) in individual semen parameters is analyzed in the statistical model based on the mean of the samples and accounting for duration of abstinence, fever, and interobserver variation (percent immotile spermatozoa). The greatest changes in intraindividual CV% of semen parameters are most marked within the first three semen samples for all three semen parameters. Blue diamonds = sperm concentration; red squares = percent immotile spematazoa; black triangles = percent normal spermatozoa. Carlsen. Longitudinal semen variation. Fertil Steril 2004. Fertility and Sterility 2004 82, 358-366DOI: (10.1016/j.fertnstert.2004.01.039)