By contrast those with blood type A seemed to have more and better quality eggs.
The findings could lead to women with type O blood being advised to try for a baby earlier, but experts said more research was needed before such a step was taken.
The blood groups O and A are the two most common in the British population: some 44 per cent are type O and 42 per cent are type A.
The lead author, Dr Edward Nejat, of the department of obstetrics and gynaecology at the Albert Einstein College of Medicine in New York, said his findings were based on women having fertility treatment at the Yale University IVF programme and the Montefiore Institute in New York.
He is presenting his findings tomorrow at the annual American Society for Reproductive Medicine (ASRM) conference in Denver.
The study of 560 women, whose average age was just under 35, found that those with blood type O were more likely to have higher levels of "follicle stimulating hormone" (FSH) than those with type A.
Fertility experts regard a high FSH level as a key indicator of having a low egg count. FSH is produced by the body to stimulate the follicles in the ovaries that produce eggs.
As a woman's ovaries run out of eggs in her 30s and 40s, production therefore has to be stepped up to encourage more eggs. The presence of high levels of FSH indicates lower numbers of eggs.
The study found that women with blood type O were twice as likely to have an FSH level above 10 – regarded as the threshold between normal and raised levels – as those in any other blood group.
People with blood group A carry the A antigen, a protein on the cell surface, that is absent in people with O type.
Dr Nejat said: "Those with blood type O were twice as likely to have an FSH level over 10 than those with blood types other than O. We found that women with the A blood group gene were protected from this effect."
Susan Seenan, of Infertility Network UK, welcomed the study. "Anything that might help couples avoid having fertility treatment has got to be good news," she said.
Tony Rutherford, the chairman of the British Fertility Society, said the research was "interesting".
But he said a study needed to be carried out among the general population, not just those who had gone for help with fertility problems.