Almost one in five girls who are sexually active at the age of 18 say they have been pregnant at least once, according to a major government study published today.

The figures were even higher among those from the poorest families and those with the lowest educational achievements. Some 28% of all girls who are eligible for free school meals reported getting pregnant at least once, with 7% having done so twice or more.

And a third of those who left school with four or fewer GCSEs at grades D-G had been pregnant at least once by the time they were 18. The figure among those who scored eight or more GCSEs at grades A*-C was only 6%.

The statistics – part of wider research into the experiences of thousands of 18-year-olds in England published by the Department for Education – found that of all those who had been pregnant, just under half (46%) had given birth, while more than a third (36%) had an abortion. The remaining 18% had a miscarriage.

Some 83% of girls were having sex by the age of 18. Of those, 18% had been pregnant at least once. The proportion among all girls, including those who were not sexually active, was 15%.

Researchers from the Youth Cohort Study and the Longitudinal Study of Young People in England, which questioned 14,800 young people, noted that the likelihood of pregnancy appeared to be linked with parents' jobs and levels of education, and that there was a "notable trend" between academic attainment and pregnancy.

Of those girls whose parents did not have A-levels, and those who parents worked in routine occupations, the level ran as high as 20%.

Among all the girls who got pregnant, 21% did so more than once.

Britain has the highest teenage pregnancy rate in western Europe. But the number of teenage pregnancies in England and Wales fell by 4% in 2008, according to figures released by the Office for National Statistics (ONS) in February.

A total of 41,325 women under 18 fell pregnant in that year, compared with 42,988 in 2007. Of those, almost half had an abortion. For every 1,000 girls aged between 15 and 17, there were just over 40 pregnancies.

Gill Frances, the chair of the Teenage Pregnancy Independent Advisory Group, said that although figures were at their lowest for 20 years, there was "still an unacceptably high" number of teenage pregnancies in the UK.

"We need to do more to help young people get easy access to reliable long-acting contraception and to identify and support those most at risk of early pregnancy," she said.

The statistics, she added, were a reminder that it was "critical to maintain investment in young people's contraceptive and sexual health services" despite public spending constraints.

A DfE spokesman said that although the recent ONS figures had shown an "encouraging decline", there was clearly "still much more to be done to encourage young people to make safe and healthy choices."