A woman's blood group could influence her chances of getting pregnant, scientists have found.
Those with blood type O may struggle to conceive due to a lower egg count and poorer egg quality, while those with blood group A seem to be more fertile.
More than 560 women with an average age of 35 undergoing fertility treatment took part in the research, led by experts from Albert Einstein College of Medicine in New York and Yale University.
Blood samples were taken to measure levels of follicle stimulating hormone (FSH), a known marker of fertility.
FSH levels greater than 10 suggest a woman will have more difficulty conceiving than those whose levels are under 10.
A high FSH level indicates a diminished ovarian reserve, which refers to both egg quality and the number of eggs left available for fertilisation.
Ovarian reserve tends to decline significantly as a woman reaches her middle and late 30s and faster in the early 40s.
The study found that women who were blood type O were twice as likely to have an FSH level greater than 10 as those in any other blood group. The findings held true even when a woman's age was taken into account and the fact the women came from two different clinics.
Meanwhile, those with blood group A were "significantly less likely" to have an FSH level greater than 10 than those who were blood group O.
Some 44% of the UK population are blood group O and 42% are type A.
People with blood group A carry the A antigen, which is a protein on the surface of the cell, but this is absent in people with O type.
Dr Edward Nejat, from the department of obstetrics and gynaecology at Albert Einstein College, is presenting his findings at the American Society for Reproductive Medicine (ASRM) conference in Denver.
He said: "In both groups of women that were seeking fertility treatment, 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 A and AB – women with the A blood group gene – were protected from this effect of diminished ovarian reserve.
"From the population we studied, and the fact it was two different centres and there was a good mix of patients ethnically and racially, we can say that blood type O was associated with an FSH level greater than 10 in women seeking infertility evaluation and/or treatment.
"Patients with blood type O seeking infertility evaluation at these centres have a higher likelihood to be diagnosed with elevated FSH and hence manifest diminished ovarian reserve."
Nejat said FSH levels were just one marker of fertility and more studies were needed."A woman's age remains the most important factor in determining her success of conceiving.
"The baseline FSH gives us an idea of the quality and quantity of a woman's eggs."
Tony Rutherford, chair of the British Fertility Society, said: "This is the first time that I'm aware of that researchers have shown a link between blood group and potential for fertility."
However, he said there were other hormones that predicted diminished ovarian reserve which were also important to assess.
"This is interesting and it shows a potential link but we really need to look at it with these other, more up to date tests of ovarian reserve."
Rutherford said a bigger study would need to be carried out in the general population to see if blood group caused potential problems for all women trying to conceive.
"We need to look at a prospective group of women to see if blood group affects your chance of getting pregnant," he said. "This needs further exploration."
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