Nothing matches mother's milk for nourishing healthy children, but Chinese moms face many obstacles to breast-feeding - the rush to get back to work, cultural norms and aggressive baby formula sellers.

When she hears her hungry baby wail, Zhang Shuyi is at her wits' end. Although she was confident she could breast-feed her baby properly, she now fears she doesn't have enough milk.

After giving birth to her son two years ago, 35-year-old Zhang, a doctor at the Capital Institute of Pediatrics in Beijing, has continued to practice exclusive breast-feeding in strict accordance with international standards.

"Exclusive breast-feeding" is defined as feeding an infant only breast milk for the first six months of life, then continuing to breast-feed as a supplement to increasing amounts of solid food for at least the first two years of life.

Although her son looks healthy, he is somewhat lighter and slimmer than other babies his age, which has concerned both Zhang's and her husband's parents. Older Chinese often believe that it is good for infants to be fat. Her parents have urged her to switch to formula.

Only a small proportion of Chinese mothers undertake exclusive breast-feeding. Before 2007, China's definition of exclusive breast-feeding allowed mothers to also give water, making it different from the international definition. Therefore, China lacks representative statistics about the rate of true, exclusive breast-feeding before that year.

David Hipgrave, chief of health and nutrition at UNICEF China, says that only a few areas of China have assessed exclusive breast-feeding rates before 2007. In one study in east China's Zhejiang Province, the rate of truly exclusive breast-feeding for six months was extremely low. The highest rate in rural infants was only 7 percent, whereas the rate in urban infants was below 1 percent.

But, some experts estimate that the rate stands at less than 30 percent currently.

Many studies have shown that breast milk is the most hygienic and nutritious food for infants during the first six months of life. After six months, when babies begin to eat solid food, breast-feeding should continue for up to two years and beyond because it is an important source of nutrition, experts say.

There is evidence that breast-feeding protects infants against sudden infant death syndrome (SIDS), allergies, acute respiratory infections and ear infections. A WHO-led global study shows that adults who were breast-fed as infants had lower blood pressure and cholesterol, as well as a lower prevalence of obesity and diabetes.

"Although I have a lot of knowledge about breast-feeding, I still worry whether my milk is enough for my baby," Zhang says.

Zhang's husband, Liu Huan says, "I received some child-rearing training before my son was born. I know that breast milk is the best for our child, so I always encourage my wife to breast-feed our baby. Sometimes, it's very hard for her."

Zhang says that the first month after her baby was born, as well as the time she returned to work after maternity leave, were the hardest times for her to breast-feed. During the first month after her child's birth, she worried that she couldn't feed him enough. After resuming work, she became very tired, which also affected her ability to produce milk.

Zhang continued to breast-feed her baby after he reached his first birthday, a time when most Chinese mothers choose to wean their children. "Many people have criticized me. I do remember one time, this old woman saw me feeding my son and said, 'What a fortunate child.' I felt so happy to hear that - if only more people could give us encouragement," she says.

Many of the children who were born during the emergence of China's one-child policy are now parents themselves. Many undertake breast-feeding at first and they understand that mother's milk is best. But they have faced difficulties.

An Jiangqun, 32, was still taking work calls on her way to the hospital delivery room. After her daughter was born, she found herself more tired than ever before. Her seven-month-old daughter drank formula on the day she was born because An couldn't produce any milk.

However, An has breast-fed her child every day since then, even hiring a nurse to give her breast massage to help her produce more milk. When An resumed her work after her maternity leave, she had little time for breast-feeding.

"I've been a bit indecisive about continuing breast-feeding, especially since I've been feeling so much pressure from work. But I've managed to persist," she says.

She relocated to a new home closer to her office and purchased an electric bicycle, allowing her to make quick trips during her lunch break to breast-feed her baby. On days when she can't make it home, she carries a breast pump and an ice chest to collect and store the milk for her baby to drink later. Her goal is to keep breast-feeding through her child's first year of life.

"Although my daughter is slimmer than other babies her age, she could stand up on her own much earlier than others. She plays by herself and doesn't like to cry," An says.

An often browses online parenting forums, encouraging other parents to practice breast-feeding and swapping tips with other mothers.

Hipgrave says that breast-feeding is not always convenient in China. Many women often regard it as very private and find it difficult to breast-feed in public places. Few public places have private nursing rooms.

Women who reenter the workforce often find it difficult to continue breast-feeding. Still others believe they cannot produce enough milk to feed their children properly, although studies have shown that virtually all women are capable of producing a sufficient amount of milk.

Hipgrave notes that advertisements for baby formula are pervasive and says sales promotions for breast milk substitutes have shaken the determination of many young mothers to breast-feed.

An says milk powder companies got her number and pester her with calls and promotions. During public parent-child activities and holidays, the companies set up booths and hand out gifts.

Although China issued a regulation to prohibit the advertisement and promotion of breast milk substitutes in 1995, the regulation is not well enforced.

Promotions are mainly targeted at mothers in hospitals, supermarkets and malls, according to the China Consumers Association, which studied 15 cities.

Experts say formula milk powder makes children more prone to infections, asthma, obesity and diabetes. Low-quality milk powder and formula substitutes are often marketed in poor rural areas, where many parents have been forced to leave their children in the care of their grandparents while they work in the cities.

Although the breast-feeding rate in remote rural areas is much higher than that in cities, rural people have many misunderstandings regarding breast-feeding.

Ye Yan, a village doctor in Baizi in southwest China's Yunnan Province, says more than 90 percent of the women breast-feed until their children are 1 year old.

But she says they are also encouraged to add food as early as the fourth month.

For remote and poor regions, another problem is more pressing. Malnutrition is common among infants over the age of six months as a result of the improper introduction of non-nutritious supplementary food.

According to a 2009 UNICEF report on nutrition, China is home to 13 million of the world's 177 million stunted children. Although rates have declined, China still has more stunted children than any country in the world, except for India. Most of these children live in poor rural areas.

Chen Chunming, an expert with the China Disease Prevention and Control Center, says many children under age of two suffer anemia because most traditional Chinese diets lack fresh meat, which leads to iron deficiencies.

The rate of anemia among urban infants is over 20 percent, and the figure is higher among rural infants, Chen says.

UNICEF is cooperating with the Chinese government to provide food supplements to address nutritional problems.

Families in 550 rural counties will be provided with free daily supplements, which contain a mixture of nine vitamins and minerals in a soybean-based powder. Counties in the provinces of Gansu, Shaanxi and Sichuan have already seen a 30 percent reduction in new anemia cases since supplements were introduced in 2009.