A recent study, led by experts from Yale University and the Albert Einstein College of Medicine in New York, found that women with blood type O may have more difficulty becoming pregnant. Women with blood type A seem to have superior protection against falling egg counts.

Experts believe that this is due to the lower egg count and poorer egg quality of blood type Os.

Researchers took blood samples from over 560 women undergoing fertility treatment and measured levels of follicle-stimulating hormone (FSH), which is a marker of fertility. High FSH levels—greater than 10—indicate diminished ovarian reserves. This refers to egg quality and quantity left for fertilisation. Women with high FSH levels are likely to have more difficulty conceiving than those with levels below 10.

Ovarian reserves usually begin to decrease around the late 30s, speeding up in the early 40s. Even when age was controlled for, researchers found women with blood type O to be twice as likely to have FSH levels over 10. Women with blood type A carry the A antigen, a protein that is present on the surface of the cell.

About 44% of the population have blood type O, 42% have A and 14% have AB. This kind of potent breakthrough could lead women to consider the blood types when choosing when to start a family. However, researcher Dr. Edward Nejat of the Albert Einstein College reiterates: “A woman’s age remains the most important factor in determining her success of conceiving.”