Shanghai is facing a gender imbalance due to migrant people's preference for baby boys, according to a report issued by the Shanghai Statistics Bureau yesterday.

The imbalance among babies under three has increased over the past decade. However, the imbalance in the working-age population has decreased.

The national census, conducted in November 2010, revealed that the overall gender ratio between Shanghai's male and female residents was 106.19:100, up from the 105.68:100 in the previous census 10 years ago.

Residents include those with local registered residency and migrant people living in the city for more than six months.

The gender ratio among all children under the age of three was higher, at 113.71 boys for every 100 girls among local residents. Ten years ago, it was 110.48 boys to 100 girls.

The census found that the boy-to-girl ratio among migrant people's children under three was 126.13:100, much higher than the desired norm of 103 to 107 boys per 100 girls.

The boy-to-girl ratio among people with local registered residency was 106.09:100, within the desired norm.

The severe gender imbalance among migrant children caused the increase in overall gender ratio among children under three, the report said.

"Migrant parents tend to leave girls at their hometown but take boys with them when working in the city," said Ren Yuan, professor of Fudan University's School of Social Development and Public Policy.

In addition, the unbalanced birth gender ratio of migrant people, mostly farmers who were keen to have boys rather than girls, caused the gender imbalance among young children, he said. Illegal sex-selective abortions thrived in many parts of the country, as the country had a traditional preference for sons, he added.

Meanwhile, a large number of out-of-town women have come to the city, decreasing the gender imbalance in the working population over the past decade, the report said.

The gender ratio between male and female workers 59 was 108.41:100, down from 110.47 in the previous census 10 years ago. Among migrant workers, the ratio was 117.47, down from 130.78 in the previous census.

"The increase in the ratio of women workers is caused by Shanghai's industrial structure upgrading," Ren said.

Due to the service industry development, more women were coming to the city to work in the domestic service industries, catering service industries and other social service industries.