Over half of this year's university applicants picked their sixth form subjects without considering their future prospects, suggests a poll.
And almost one in five of over 1,000 UK 18- and 19-year-olds polled for Which? said different subjects might have been better for their chosen degree course.
The consumer group says teenagers need better guidance on subject choice.
"It is a complex and difficult decision for young people," said head teachers' leader Malcolm Trobe.
The poll of 1,020 teenagers who applied to university this year also revealed that:
- 30% felt adequately informed about how their subject options would affect their university and degree choices
- and 29% would have appreciated more advice on which subjects to take.
"Make sure you do your homework and choose wisely," Alex Neill of Which? University advised students.
"If you've just received your GCSE results, and are now choosing your A-level subjects, it's important to know your options and get some advice."
- Check the exam entry requirements for university subjects.
- Some universities have lists of preferred and less-preferred subjects
- Some will consider a student no matter what subjects they might be doing
- Don't forget vocational qualifications can also lead to university.
Mr Trobe, interim general secretary of the Association of School and College leaders, said young people needed the chance to research and filter information on subject choices and university course requirements.
But he warned that young people often changed their minds between the ages of 16 and 18 - and sometimes could be caught out by the greater demands of A-levels, despite having performed well at GCSE.
He said schools needed to help with students' subject choices, particularly when the decline in the use of AS levels means less chance to try a subject and drop it after one year.
"The most important thing is not to wipe out options by an unwise choice," said Mr Trobe.
"It is really important that young people, especially those from disadvantaged backgrounds, have clear information about how the subjects they study in sixth form or college can affect their options at university," said a spokesman for the Russell Group of leading universities.
"Our universities are constantly trying to spread the word that with the right grades in the right subjects a place at a Russell Group university is well within reach. But students need good advice and information from a range of key sources."