I'm trying to find a story about humanity starting to go infertile.
The premise of the story was that human sperm contained within it another human, which contained another and so on. It has now reached the point where there were no more "included" and so it was the last level.
Scientists were working on a way to fix the problem.
This sounds very much like Seventy-Two Letters (2000), a novella by Ted Chiang.
It is in a steampunk type setting, and in this alternate reality reproduction is subject to the "law of preformation", in which sperm cells contain miniature versions of humans, whose sperm in turn hold miniature versions... and so on:
all living things had been created at the same time, long ago, and births today were merely enlargements of the previously imperceptible. Although they appeared newly created, these homunculi were countless years old; for all of human history they had lain nested within generations of their ancestors, waiting for their turn to be born.
A crisis is found to be happening, when advances in microscopy reveal that the number of nested homunculi is coming to its limit:
"We are in agreement as to the implication of these findings: that the human species has the potential to exist for only a fixed number of generations, and we are within five generations of the final one."
The scientists indeed seek a way to resolve this crisis, using techniques borrowed from golem creation.
A copy is available to read at the Internet Archive.
