About 40 percent of Beijing families that have or were going to have a second child said they did not plan for it to happen, a new survey found.

And those with two children are particularly concerned about how to deal with the siblings' relationship, according to the survey results released on Friday at the Annual Conference of the Chinese Society for the Study of Marriage and Family.

"About 40 percent of surveyed women said their second child was a surprise, in a good or bad way," said Mao Zhuoyan, a leading researcher with the National Health and Family Planning Commission's Science and Technology Research Institute.

The study, led by Mao in late 2015, collected 500 responses from 16 districts and counties in Beijing, surveying women ages 20 to 49. All of the women surveyed had or were pregnant with a second child by August last year.

It found that about one-fifth of those surveyed had an abortion between the two births.

In 76 percent of those cases, "unprepared pregnancy" was the major reason for having an abortion.

"This shows that women urgently need safe contraception," Mao said.

The survey found that the average age of women in Beijing during their second delivery is 33. About 27 percent gave birth a second time when 35 or older.

The top concern of the mothers of two children is how to deal with the siblings' relationship, according to the survey.

Most families in China had only one child before the country eased its family planning policy on Jan 1.

"Whether the elder child will become jealous is a practical problem that many families face," said Wang Yuqiong, director of the nursing department of Chengdu Women's and Children's Central Hospital in Sichuan province.

The hospital has opened a counseling clinic for people who want to have a second child, according to China News Service.

Wang suggested that parents pay enough attention to psychological changes in the first child to avoid tensions in the siblings' relationship.