China has broken up a gang that was providing illegal sex-determination tests, largely for gender selection, the top health authority announced on Sunday.

The gang tested more than 1,000 women around China in 2013, said the National Health and Family Planning Commission.

One of the main suspects, identified as Wang Ermin, has been convicted and sentenced to three and a half years in prison, according to the commission.

The gang started offering the illegal tests in 2010, first in Yiwu, Zhejiang province, and soon expanded into most parts of the country.

They were exposed after a woman named Wu died of amniotic fluid embolism after a test organized by the ring in late 2012.

The method used — amniotic fluid extraction — is more accurate than an ultrasound, but involves far more risk, a medical expert said.

According to Wu's husband, they traveled from Yiwu to Zhengzhou, Henan province, for the procedure, which cost around 2,000 yuan ($330).

The couple, who already had a 2-year-old girl, were desperate to have a boy who would bring more economic benefit to the rural family during a compensation program.

Breaking up the gang "was a major achievement under the one-year campaign launched last October by government agencies including the commission and the Ministry of Public Security", the commission said.

In China, sex determination testing, gender-selective procedures and abortions for non-medical purposes are prohibited, particularly given a traditional preference for boys over girls and a largely skewed sex ratio, which stands at 115 boys born for every 100 girls.

Internationally, a normal range should be between 103 and 107. "Sex-selective abortions after sex determinations have further fuelled an imbalance," said Lu Jiehua, a sociology professor at Peking University.

Zhai Zhenwu, a professor at the School of Sociology and Population Studies at Renmin University of China, agreed, adding that an increasing access to ultrasounds since the late 1980s has made sex selection far easier and has partially led to an imbalance in the sex ratio at birth.

In 2004, it hit a record 121.2 boys for every 100 girls.

In response, some population scientists called for making it a crime to perform sex determinations and sex-selective abortions that cannot be medically justified

But some argue that it is a personal right to learn the gender of one's fetus.

Also, it would be hard to collect evidence as the doctor could imply the gender during a normal pregnancy test, said Zhai.

He said that if harsher punishments had been meted out, the growing imbalance would have been curbed better.

Most countries allow sex determinations as "it makes no difference," he noted.

In China, "the preference for boys became more intense as the three-decade-old family planning policy restricted most families to just one child," he said.

Thereafter, more people resorted to artificial methods to have a boy due to the rising availability of ultrasound technology, he said.

But he refuted a causal relation between China's birth rule and a skewed sex ratio, citing the example of South Korea.

Sex determinations are outlawed in South Korea, which recorded a high gender ratio at around 117, according to Zhai.

"But they never had birth rules," he added.

The root cause lies in a slow-changing preference for boys, he said.

In recent years, some rich Chinese have gone to neighboring Thailand to undergo sex-selective procedures enabled by assisted reproductive technology to have a boy, previous reports said.

In China, such a practice is only available for medical purposes like testing for genetic diseases, according to fertility experts.

According to Zhai, the constant relaxation of the birth rules could help reverse the skewed sex ratio somewhat but "it will still take time for a natural transformation," he said.

Before that, government-led administrative measures should be in place to curb the trend, he stressed.

The ratio has continued to decline during the past five years thanks largely to government action, statistics from the commission showed.

Physicians caught violating such regulations face revocation of their licenses and the hospitals involved face harsh administrative penalties.