BEIJING - Upon hearing her baby wail, Zhang Shuyi finds herself at her wit's end. Although she was previously confident that she would be able to breastfeed her child properly, she worries that she might not be able to give her baby 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 breastfeeding in strict accordance with international standards.

"Exclusive breastfeeding" is defined as feeding an infant with only breast milk for the first six months of life, then continuing to breastfeed 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 slimmer than other babies his age, which has displeased 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 truly exclusive breastfeeding. Before 2007, China's definition of exclusive breastfeeding allowed mothers to also give water, making it different from the international definition. Therefore China lacks representative statistics about the rate of true, exclusive breastfeeding before that year.

David Hipgrave, chief of health and nutrition at UNICEF China, says that only a few areas of China have assessed exclusive breastfeeding rates before 2007. In one study in East China's Zhejiang province, the rate of truly exclusive breastfeeding 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, breastfeeding should continue for up to two years and beyond because it is an important source of nutrition, experts say.

There is evidence that breastfeeding 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 breastfed as infants had lower blood pressure and cholesterol, as well as a lower prevalence of obesity and diabetes.

"Although I have obtained a lot of knowledge about breastfeeding, 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 breastfeed 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 breastfeed. 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 breastfeed 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.