This week begins China’s annual mass pilgrimage, as hundreds of millions of people pack the trains and highways to return to their home towns for the Chinese New Year holiday known as the Spring Festival.

But for one particular group — young urban married couples who grew up as only children — the yearly ritual can also mean tough decisions, sometimes-painful arguments and a modern-day test of one of China’s centuries-old family traditions.

These young couples are part of the generation of only children born during the 34 years of China’s “one-child policy.” Following the typical pattern, they migrated to the larger cities from the outlying provinces to go to university. They stayed for work and then got married.

And now they must decide which set of parents to visit. It’s a decision fraught with emotion, especially for China’s growing elderly population, often living alone and far from their children, who historically have been caregivers in a country with little social safety net.

“Both of us want to go back to our home to celebrate Chinese New Year,” said Lin Youlan, 30, a government worker who married her husband, Li Haibin, 33, four years ago. “We always fight about this problem.”

She is from Chongqing in southwest China, and he is from Shandong, on China’s east coast. They live in Beijing, and they are only children.

As the only son, Li is under intense pressure to visit his parents, who are not in good health. “In Shandong province, men must celebrate the Spring Festival with their own families. And the wives should spend the Lunar New Year at their husbands’ homes,” he said. “I worry how others will look at my parents if I don’t go back home every year.”

Traditionally, the Lunar New Year’s Eve and the first day of the new year — which are Jan. 22 and 23 this year — were spent at the home of the husband’s parents, and the second day was spent with the wife’s. But in those days, married couples largely came from the same village or town or a relatively short distance apart.

Now China’s 1.3 billion people are mobile and rapidly urbanizing. The government announced Tuesday that the country’s urban population had surpassed those living in rural areas, although just a quarter of the population lived in cities in 1990.

That shift, coupled with the one-child policy and other societal changes, has left tens of millions of elderly people living alone, often with little in the way of government aid. China has few nursing homes and no tradition of professional caretakers to look after the elderly when they become infirm.

China has 178 million people who are 60 or older, according to government census figures. Li Liguo, the minister of social affairs, said that number will jump to 216 million, or 16.7 percent of the population, by 2015. At that time, Li said, there will be 51 million “empty nesters” 65 or older and living alone.

And as the older population is growing, China’s current birthrate — 1.54 children per woman — is considered far below the normal replacement rate, which is two children per woman. (The rate in the United States is 2.06).

“The elderly in the countryside is really worrying,” said Therese Hesketh, a professor of global health at the University College London who has studied China’s population policies. “Even in the U.K. at Christmastime, this is an issue that comes up,” with smaller families and couples deciding whose parents to visit, Hesketh said. “This is a universal issue magnified in China by the one-child policy.”

Some Chinese couples try to resolve the annual conflict by visiting both sets of parents. Chen Juan, 29, and her husband, Huang Feilong, 31, met in Beijingthrough an online dating site. They were are both from Hunan province, from cities about three hours’ drive apart. They got married in 2008 and have spent four Chinese New Years together — three at his parents’ home and one with her family. “We fight about this almost every year,” Chen said.

This year, they are dividing the weeklong holiday in half, the first and most important days with his family, then the remainder with hers. But China’s size — as well as the difficulty of finding bus and train tickets over the holiday period — makes traveling to two sets of parents impractical for many.

Chinese economists and academics have recently been engaged in a vigorous and surprisingly public debate over whether it is time to scrap the one-child policy and allow couples to decide for themselves, with some pointing to the empty nester problem as a reason to make a change.

But so far, the central government has shown no sign of changing the policy. Li Bin, head of the National Population and Family Planning Commission, said last year that China’s growing population remained a challenge and that the government would continue the policy.

The policy actually only covers about 35 percent of Chinese, mostly those living in urban areas, with exemptions granted many others. Farmers are allowed to have more children, for example, and members of ethnic minority groups are exempted.

The law was implemented in 1978 to control overpopulation and the strain on scarce resources. Authorities estimate that the policy has prevented 400 million births, and it is credited with helping lift the country out of poverty.

But the law also has been severely criticized. There were reports of some provinces forcing women who became pregnant in violation of the policy to undergo late-term abortions or sterilizations. There were earlier stories of female infants being killed because of a preference for boys. China now has a gender imbalance due to the policy.

The law has also been said to hit the poor harder. Some wealthy families have as many children as they want because the penalty is a fine that most can easily pay. And some economists say the future labor shortage China is facing is another reason to scrap the policy.

Others here have argued that an increasingly urban lifestyle makes a one-child policy obsolete. Most couples would probably choose to have just one child, they say, because the cost of raising children is so high.

But some young couples who see their own struggles with such matters as visiting relatives over the holidays say they are committed to having more than one child.

“I want two children in the future: one boy and one girl,” Chen Juan said.

Staff researcher Zhang Jie in Beijing contributed to this report.