An employee arranges a test tube in a container used to freeze human eggs for in vitro fertilization at a fertility clinic in Greece. (Kostas Tsironis/Bloomberg)

A long-established San Francisco fertility clinic experienced a liquid nitrogen failure in a storage tank holding thousands of frozen eggs and embryos for future use, jeopardizing tissue hundreds of women had stored in hopes of having children.

The March 4 incident at Pacific Fertility Clinic, acknowledged Sunday by the facility’s president, followed a similar malfunction the same weekend at an unrelated clinic in Cleveland, the University Hospitals Fertility Center.

The two episodes carry powerful emotional and financial consequences, and come as the number of women freezing their eggs has soared in recent years. Although individual women have reported having frozen eggs damaged in storage or in transit, a spokesman for the American Society for Reproductive Medicine, a major professional organization, said such large-scale incidents appear to be unprecedented.

“We can’t say definitively nothing like this has ever happened, but we are certainly not aware of anything,” said Sean Tipton, the association’s chief policy, advocacy and development officer. “Now that we have a second incident, it becomes very important that we learn as much as we can about both, to search for commonalities and see if there are . . . risks that have now come to light that need to be addressed.”

Officials at Pacific Fertility said the latest problem was discovered by the clinic’s laboratory director, who noticed during a routine check that the level of liquid nitrogen in one of the clinic’s steel storage tanks had fallen too low. Too little liquid nitrogen causes the temperature in the waist-high tanks to rise, risking damage to tissue housed in vials called cryolocks. Each vial can contain as many as three eggs; embryos — fertilized eggs — are stored individually.

The clinic declined to say how many eggs and embryos had been affected but said the malfunctioning tank, storage tank No. 4, contained “several thousand” eggs and embryos. Clinic spokesman Alden Romney said that represents as much as 15 percent of the total stored at the facility.

In an interview, the clinic’s president, Carl Herbert, said the lab director immediately transferred the threatened eggs and embryos to a spare storage tank brought into the lab and filled with liquid nitrogen. Staff then spent days sorting through records to verify which patients had tissue inside.

On Saturday night, the half-dozen doctors who work at Pacific Fertility were finally able to begin making phone calls to some 400 patients who had all their eggs or embryos stored in storage tank No. 4.

Early Sunday, the clinic sent out emails notifying two other groups: Roughly 100 patients whose tissue was split between tank No. 4 and other tanks, and a larger group whose tissue was unaffected.

Herbert said his discussions with patients were emotional. “Anger is a big part of the phone call,” he said. “Our goal is to provide all the patients we see with some kind of a family. . . . We need to think: If this tissue doesn’t work, what are the next steps, and have you not feel defeated.”

Herbert said the extent of the damage is not yet clear. When clinic staff thawed a few embryos affected by the malfunction, they found that the tissue remained viable. Staff have not checked any of the eggs, he said.

“This was a terrible incident,” Herbert said, “but I was reassured that . . . [staff] did everything anybody could ever want to do.”

The clinic has reported the incident to the College of American Pathologists, which certifies labs, and the overseers of California’s tissue banks, Herbert said. The clinic also has brought in a multidisciplinary team to investigate the malfunctioning tank and, he said, “every aspect that involves cryopreservation.”

Some of the eggs and embryos in tank No. 4 had been in storage for as long as 10 years, Herbert said, though the tank also was still in active use. Herbert said the clinic does not yet know how many affected patients had been planning to use the stored tissue.

According to its website, Pacific Fertility’s fees for egg-freezing start at $8,345 for an initial cycle and $6,995 for subsequent rounds. In each cycle, a patient’s ovaries are stimulated with drugs to produce multiple eggs, which are then harvested by doctors and frozen.

For affected patients who are still eager to use their eggs or embryos to try to become pregnant, Herbert said the clinic plans to first thaw them and check for viability. If the tissue is not viable, he said, “we are going to make our patients happy one way or another.”

Herbert is a longtime physician and researcher in assisted reproductive technology. In 1982, he helped develop one of the nation’s earliest reproductive technology programs at Vanderbilt University Medical Center. He moved to San Francisco in 1990 and, with colleagues, purchased Pacific Fertility Center nine years later.

In the earlier incident in Cleveland, officials at University Hospitals Ahuja Medical Center’s fertility clinic notified about 700 patients that their frozen eggs and embryos may have been damaged. Some had been in storage since the 1980s.

On Thursday, hospital officials said in a statement that they were investigating the incident and that it was unknown whether the cause had been human error or mechanical failure.

Last week, the American Society for Reproductive Medicine issued a statement calling the Cleveland incident a “tragedy.” It said, “First and foremost our hearts go out to the patients who have suffered this loss.”

As assisted reproductive technology has advanced, the services of fertility clinics — and therefore egg- and embryo-freezing — have become increasingly popular. In addition to patients who freeze eggs and embryos while trying to become pregnant, a rapidly growing number of American women are choosing to freeze their eggs as a hedge against later infertility.

The number of egg-freezing patients jumped from 475 in 2009 to 7,518 in 2015, the most recent year for which figures are available from the Society for Assisted Reproductive Technology. In total, about 20,000 American women have had their eggs preserved.

In 2016, the American Society for Reproductive Medicine, the Society for Assisted Reproductive Technology and the Society for Reproductive Biologists and Technologists developed joint guidance, recommending that all in vitro fertilization programs have a plan to protect the human tissue they keep “in the event of an emergency or natural disaster.”

The emergencies mentioned in the guidance are fires, floods, power failures and terrorist attacks — but do not include instances in which a clinic’s own equipment fails.

Correction: An earlier version of this story should have said that staff at Pacific Fertility Clinic had thawed a few embryos affected by the malfunction but had not checked any eggs.

Ariana Eunjung Cha contributed to this report.