To determine whether a clinically-relevant change in the total motile sperm count (TMSC) over time exists within the subfertile population.

The first semen analysis of all men presenting to selected infertility centers in 2 countries between 2002 and 2017 were evaluated. Semen analyses were categorized into 3 clinically-relevant groups based on treatment options: TMSC >15 million (M) (Group 1), in which no insemination intervention would be required; TMSC 5-15 M (Group 2), in which intrauterine insemination would be appropriate; and TMSC of <5 M (Group 3), in which in vitro fertilization would be considered. Relationships between male age, TMSC, trend of TMSC, and TMSC group membership by year were assessed.

A total of 119,972 first semen analyses were included. The proportion of men with normal TMSC (>15 M) was found to decline approximately 10 percentage points over the past 16 years in the analysis of combined centers (odds ratio 0.967; 95% confidence interval = 0.963-0.971; P = 2.2e-16). A reciprocal increase was distributed between both the moderate (5-15 M) and severe (<5 M) oligozoospermia groups. Additionally, TMSC declined 1.1 percentage points with each year of advancing paternal age. No difference was seen in age at presentation by year.

The proportion of men with normozoospermia declined and that of men at risk of requiring fertility treatment increased over the study time period. Although several unknown factors may have influenced our data as a result of the retrospective design, a shift in treatment group membership over time may be clinically relevant.