As Japan’s birthrate declines, a Tokyo suburb is unrolling a new perk to encourage more women to have babies. Over the next three years, the city of Urayasu will help cover the cost of egg freezing, a fertility treatment that has recently gained traction in the United States.

A university hospital will run the project, according to the Associated Press. Eligible women, 24 to 35, would still have to pay 20 percent of the expense, which could reach the equivalent of $2,000. So far, a dozen women have accepted the city’s offer.

Egg freezing is billed as a way to stop the biological clock — but it’s not that simple. Although doctors are generally confident that egg freezing can increase a woman's chances of pregnancy above a certain age, the very pricey process is far from guaranteed to produce a baby. Whether it's capable of replenishing a city's population is highly uncertain.

Doctors say fertility sharply declines for women as they approach 40. The finite supply of eggs set at birth has considerably depleted, and those remaining weaken with time, becoming less optimal for pregnancy.

Fertility specialists recommend egg freezing to women who have reached at least age 30, want children and haven’t met a compatible partner (or don’t have the financial means to find a sperm donor and raise a kid on their own).

When to freeze, however, remains a gamble. Some physicians believe a 30-year-old woman’s chances of meeting a mate by the time she’s, say, 34, are pretty good — so spending what amounts to be $10,000 in the United States could be a colossal waste of money. Wait too long to freeze, however, and eggs are less likely to become fertilized.

Whether the procedure will actually work is also highly uncertain. The chance one frozen egg will lead to a baby, even in younger women, is between 2 percent and 12 percent, according to the American Society for Reproductive Medicine.

A 2015 study found much better odds. In one sample of 2,227 frozen eggs, the live birth rate was 43.2 percent. (The live-birth rate for fresh eggs in an IVF cycle was slightly higher at 49.6 percent).

Earlier this year, Japan hit an unhappy milestone: The country’s official census reflected for the first time in modern history its population had actually shrunk. The 2010 census showed a population of 128,057,352. The 2015 number, released in February, was 127,110,000.

The country needs a birth rate of 2.1 per woman to sustain growth. Currently, that figure rests at 1.4 per woman.

Three years ago, Japanese Prime Minister Shinzo Abe attempted to tackle this problem by calling for more gender equality in the workplace. He also pledged to create 400,000 new day-care spaces nationwide by 2018 and doubled the government-provided “child allowance.”

The birth rate isn’t exactly soaring in the United States, but economists here don’t see that as a cause for alarm. Births grew by 53,743 from 2013 to 2014, the first increase in seven years, following a period of dips since the recession, according to the National Center for Health Statistics.

Egg freezing in the U.S. is seen by some employers as a worker retention tool.

The closest thing we have to government-funded egg freezing comes from the military. In February, the Pentagon announced it will pay for service members to freeze their eggs or sperm.

Defense Secretary Ashton B. Carter said at the time he hopes the program will encourage women to stay in the military during their peak childbearing years. It would also serve as fertility insurance for veterans. If they’re wounded in combat and become unable to have children, they could turn to the frozen cells.

The Pentagon, one of America’s largest employers, follows a handful of prestigious companies that offer the perk. Apple and Facebook, for example, both offer to pay for a portion of the expense for employees. So do Citigroup and JPMorgan.

Without employer-provided benefits, most Americans have to pay out of pocket for egg freezing. Insurance companies don’t typically cover the procedure.

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