Sperm counts are just the start. Motility and morphology are also factors. Motility is the percentage that can swim forward. Morphology is the percentage that

Sperm counts are just the start. Motility and morphology are also factors. Motility is the percentage that can swim forward. Morphology is the percentage that are normally shaped. Using the widest ranges, normal morphology is 40% or better, and normal motility is 25% or better. So, the total count only tells us so much. The number that are actually suitable for conception are a subset of the total, and if the total goes down, the subset becomes smaller. The lower the number of quality sperm, the harder pregnancy becomes. Contra other commenters in this thread, the extra time this requires is probably not a linear function but geometric. At a certain point, outside intervention is most likely required (IUI/IVF).