Fully 46 percent of the more than 20,000 girls who called the city's pregnancy hot line over the past two years said they had sex with boys they met online, the China Daily said, citing Dr. Zhang Zhengrong of Shanghai's No. 411 Hospital.
Most of the would-be fathers disappeared after being told of the pregnancies, while in some cases the girls did not even know their partners' true names, the report said.
The report said calls to the hot line have shot up 12 percent since the start of school holidays, which started two weeks ago and run through August.
Earlier reports have cited a 30 percent increase in abortions by teens during holidays, with high school students between 16 and 18 accounting for a growing percentage of those seeking to end their pregnancies.
Zhang said callers to the hotline generally knew little about birth control or the physiological aspects of sex and widely considered abortions to be harmless.
About 10 percent who called had undergone multiple abortions, while "there were some who were unaware they were even pregnant until very late," Zhang was quoted as saying.
While underage sex remains taboo in China, abortion is widely available without the requirement that parents be notified. China has long promoted abortion as part of its attempts to enforce policies limiting most families to just one child.
Zhang said 79 percent of high school and university girls cited the Internet as their main source of information about sex in a survey conducted by her hospital.
Just 7.9 percent of parents discussed sexual matters with their daughters, she said, while 46 percent of parents said it was the responsibility of schools to provide sex education. The survey gathered results from 2,043 parents, 2,680 teachers and 1,577 teens.