The hardest part of Dr. Zev Williams’s job as a fertility physician is telling a man: “You’re not going to be a father.”

Male infertility contributes to up to 40 percent of all infertility cases worldwide. While a typical semen sample contains hundreds of millions of sperm, men with azoospermia, which affects about 10 percent of all infertile men, have zero or very few sperm in a sample.

Some cases are known as obstructive, in which a fixable blockage prevents sperm from getting into semen. Other cases are nonobstructive, where the biological machinery for generating sperm is broken, and the chances of fathering children plummets.

“That is really one of the most heartbreaking, devastating and surprising diagnoses you can give to patients,” Dr. Williams, director of the Columbia University Fertility Center, said.

Lately, however, he is sharing encouraging news with hopeful dads. For men with extraordinarily low sperm counts who have been considered azoospermic, sometimes several “hidden” sperm are present. Analyzing a sample under a microscope with the human eye often misses these individual sperm cells. Now, some of these men are benefiting from a novel technique developed by Dr. Williams and his team at Columbia called the STAR system.

Much in the same way that astronomers use machine-learning algorithms to uncover new objects in the night sky, STAR — Sperm Tracking and Recovery — uses artificial intelligence to locate difficult-to-find sperm.

Samples of semen are introduced by syringe into a microfluidic chip the size of a stick of gum, where the sample flows through a channel thinner than a human hair. As it passes through the channel, a high-powered magnifying lens connected to a high-speed camera takes about 300 images per second. The whole setup is connected to a computer, where A.I. quickly assesses those images. If the A.I. tool spots a sperm cell, it prompts the computer to open a small side valve on the microfluidics channel into which the cell is captured.

“When you look with your naked eye at night, you can find a lot of stars,” Dr. Williams said. “But using a telescope reveals a ton more. That’s what this is doing. It’s giving us an ability that is beyond what a human is typically capable of.”

Usual methods to find sperm cells in suspected cases of azoospermia involve manually looking through the ejaculate under a microscope. It’s tedious work. Scanning a single sample can take about eight hours to complete, and technicians sometimes can’t find sperm, even if they’re present. That’s because ejaculate contains billions of bits of cellular debris. For men with millions of sperm in their samples, that isn’t a problem. But for men whose samples may have as few as two or three sperm cells, finding them within that cellular clutter is a fool’s errand.

Urologists might go an extra step and order an outpatient procedure called a micro-TESE to slice open the scrotum and look in the seminiferous tubules of the testes, where sperm are created, to see if they can locate sperm cells. Sometimes they do. Sometimes they don’t.

The STAR system, which took about seven years of development, rolled out officially in late 2025 at Columbia’s fertility center. An article co-written by Dr. Williams and published in The Lancet last November tells the story of a couple who experienced 19 years of infertility. Sperm were found after the 39-year-old man’s sample was run through the STAR system, and that couple now has a baby girl.

In another instance, a patient’s micro-TESE sample was searched over two days with no sperm found. The STAR system, on the other hand, found 44 sperm cells in that same sample in one hour.

Across 175 cases, the STAR system has found and recovered sperm 26 percent of the time. From there, the normal process of in vitro fertilization takes place. Dr. Williams said there was currently a pregnancy in progress that was made possible after STAR found just two sperm in a man’s semen sample.

What the STAR system can’t fix are cases of azoospermia in which men have no sperm at all — a concern that has become more prevalent over the last decade. Researchers are split as to whether sperm counts are dropping since findings of a major meta-analysis published in 2017 argued that sperm counts among Western men had halved over a 40-year period.

Shanna Swan, a professor of environmental medicine and public health at Mount Sinai school of medicine in New York City and a co-author of that study, has argued that if the trend continues, most couples might require assisted reproduction by midcentury. “Spermaxxing” is now a term du jour in the online wellness community. Other studies, like one published by urologists at the Cleveland Clinic in 2025, found no change in sperm concentrations among fertile American men over the last 50-plus years.

“The truth may still be somewhere in between, but I think more and more people, myself included, are willing to believe that there’s a problem,” said Dr. Alexander Pastuszak, a board-certified urologist and chief executive and co-founder of Paterna Biosciences.

This past year, Paterna, based in Utah, claimed that it successfully grew human sperm in the lab. According to Dr. Pastuszak, company scientists took stem cells from testicular tissue — the germ cells destined to become sperm — and added a proprietary mix of molecules and chemicals to a petri dish to coax those cells into becoming fully fledged sperm. Third parties have yet to duplicate the results, which are not yet published in a peer-reviewed journal. But it could very well represent a milestone in reproductive biology.

“It solves the problem of men who don’t make sperm,” Dr. Pastuszak said. “These men still have all the germ cell precursors. They just don’t make sperm efficiently.”

Until lab-grown sperm are available, techniques like the STAR system will continue to fill in the gaps. Currently it’s available only at Columbia University, with a dedicated team of experts operating three systems. Ultimately, the goal is to create a more automated system that other fertility centers can use.

“With zero sperm, you effectively are giving up the hope of a biological child,” he said. “With one sperm, you could envision the child, the grandchildren, the great-grandchildren. It’s just such a profound moment when you find and recover that sperm.”