Young mom Yang Yi had a tortuous time giving birth to her first child four years ago and vowed never to have a baby again. Her doctor smiled and said a woman's labor pains were easily erased by the joy of motherhood.

Yang doubted the doctor's advice but late last year gave birth to her second child, this time a boy.

"As our daughter was growing up, we thought it was time to bring her a younger brother or sister," Yang says.

"Our parents were looking forward to, and full of joy about, our newborn baby. They joked that the two grandchildren can carry on both the family names."

The 29-year-old entrepreneur and her 31-year-old university teacher husband live in Hangzhou, Zhejiang province.

Yang Yi poses with her daughter as she awaits the birth of her second child. Courtesy of Yang Yi

They are among the new generation of young Chinese couples who can choose to have two children.

A family planning policy was adopted in the late 1970s in an effort to curb the population explosion, which threatened the nation's resources and environment. Certain exemptions were always envisaged, however, including for couples who each came from one-child families. Ethnic minority groups with small populations were also exempt.

According to the the National Population and Family Planning Commission, approximately 90 million only children born during the 1980s and 1990s will enter child-bearing age in the next 10 years.

Yang is already one of these happy mothers of two children.

"When I was a kid, I always wanted an elder brother to take care of me," she says.

"It was my happiest time spending summer holidays at my grandparents', when I could play with my cousins."

China's one-child generation is often labeled as spoilt and self-centered. They are called the "population explosion", grow up lonely, depend too much on others and are not used to sharing.

Yang clearly remembers one day when her grandmother was slicing a juicy, ripe watermelon in half. Yang didn't wait for her grandmother but snatched the fruit took a huge bite herself.

"Grandma scolded me for being self-centered and disrespectful of my elders. I grew up realizing what an important lesson she had taught me," Yang says.

Despite Yang's plan for brother-sister harmony within her own young family, her 3-year-old daughter at first didn't take too kindly to her newborn brother.

"She wasn't close to us, because we have been busy at work and entrusted much of her care to a distant aunt," Yang says.

She noted her daughter didn't sleep well for several nights after the baby was home from the hospital.

She became clingy and obedient to Yang and would get jealous and impatient when her nanny helped nurse her baby brother.

Yang tries to convince her daughter how great it is to have a brother.

"I tell her she now has a faithful playmate and helper while many other children don't," she says.

"And she can play with her brother on the backseat when we drive out."

But Yang says nature is now taking its course.

"One day I saw my daughter standing by the bassinet," Yang says. "She gazed at her brother for a while and called his name gently. I think she is ready to be a good sister."

Zheng Chen, a bank official in Hangzhou, believes that competitiveness between a brother and sister is good for children. She has a 4-year-old daughter and plans to have a second baby this year.

Zheng believes children will perform better at school if they have a brother or sister to contend for parents' love and attention. Plus, they learn from each other and take care of each other, she says.

Despite the widespread desire among young Chinese women to have two children, the reality is perhaps different.

Research shows many eligible couples do not even have one child let alone two, and many young city-dwelling couples even put off marriage.

In 2007, the Chinese Academy of Social Sciences carried out research on only-children born between 1976 and 1986. Nearly two thirds of the respondents chose to have only one child.

"Economic capacity is one crucial factor about whether people want a second child," says Ma Xiaohong from the Institute of Population in Beijing.

The cost of education is reportedly the second largest proportion of Chinese family expenditure, after food.

Besides schooling fees, parents also pay for all kinds of special classes outside school, such as English, painting and singing lessons.

"We are not rushing to have a second child until we can afford good education for both children," says Leng Xiang. The father of a 4-month boy in Chengdu, capital of Sichuan province, and his wife are eligible to have a second child.

"My cousins all have one child and do not want another one even if they could. They want to dedicate wholeheartedly to their only-child and can't spare the extra money and effort," Leng says.

Li Yanna, who also qualifies to have a second child, has always wanted a daughter ever since her son was born two years ago but must consider the needs of the entire family.

She lives with her parents-in-law who provide enormous help raising her child.

"I will have a second child only when we can afford a decent apartment for all six of us in Shanghai. But the house prices are so high so I don't see my two-children plan working," she says.

She adds that working mothers like her also fear their careers would be affected if they become pregnant.

"A full-time mother may be an alternative for a handful of people but most women still place a great emphasis on career and self-development," Ma says.

Because children are normally entrusted to the care of grandparents, only-child couples are also considering the health of their parents before planning a second child.

"We may need a nanny. But it is difficult and expensive to find someone qualified and professional," Leng says.

Ma says that unlike rural parents, the ratio of urban parents who are willing to take care of grandchildren is decreasing because they are better educated and prefer enjoying life after retirement rather than dedicating all their time to baby-sitting grandchildren.

"And also, aging parents will rely less on their children's support as the government quickens the process of old age support from families to social institutions," she says.

She doesn't know if there will be more two-child families but predicts Chinese family units will become more diversified and continue to evolve as they have over the past century.

Between the 1900s and 1950s, most families had three or four generations living under the same roof. The extended family structure became dominant from the 70s to the mid-80s.

However, since the late 70s, when China introduced its family planning policy, the extended family unit has changed to a nuclear family consisting of a father, a mother and a child all living in one household.