A woman's blood type may yield clues to her fertility, a new study suggests. The results show that, of a group of women in their 30s who sought medical fertility help, those with blood type O were more likely than women with other blood types to have diminished ovarian reserve, meaning their ovaries had few eggs or had eggs unlikely to meet with success during in vitro fertilization procedures. Type O blood is the most common type in the United States. "I don't want the message to be that women in the healthy population should be petrified that their blood type may predict compromised fertility," said study author Lubna Pal, who researches reproductive endocrinology at the Yale University School of Medicine. But if the study's link is shown to hold up for other women, then the connection may provide a tool for earlier, more accurate fertility prognoses, Pal said. The study was published online on June 26 in the journal Human Reproduction. Babies and blood types Fertility doctors expect to see diminished ovarian reserve in patients in their late 30s and 40s, but sometimes it strikes early. "There is a subgroup of women," Pal said, who are young but whose bodies act as if they're older. "There is a lesser number of eggs in those ovaries." Researchers measure levels of follicle-stimulating hormone (FSH) to see if a woman may have diminished ovarian reserve; high levels usually indicate that she does. Although most young, healthy women do not get their FSH levels measured during routine checkups, women visiting fertility centers do. Pal and her colleagues studied the FSH levels of 544 women, whose average age was 35, seeking fertility help in Connecticut and New York. After taking into account the effects of age, they discovered that women with blood type O were twice as likely as those with blood types A and AB to have FSH levels high enough to indicate they had diminished ovarian reserve. Women with blood types A and AB were generally less likely than others to have FSH levels indicating diminished ovarian reserve, the study showed. There were too few women with blood type B in the study for the researchers to determine statistically if their ovarian reserve was affected. For young women with type O blood, "maybe routine testing should be the way to go," to determine their risk of fertility problems later in life, Pal said. How and why? Because Pal's study included only women seeking fertility treatment, she cautioned that the findings of her study do not apply to the general population. However, "having said that, we have enough reasons to be worried about high FSH levels in someone who's not infertile," she said. If a young, healthy woman came in to her office and had elevated FSH levels, "I would be testing that woman" for other signs of decreased fertility, she said. Researchers need to determine whether the relationship between blood type and fertility is consistent across national and global populations, and if it exists in healthy women, Pal said, then scientists need to ask: "How does it work?"
Pal hypothesized that the same molecular machinery that determines blood type could affect ovarian cells. Red blood cells have an identifier molecule that sticks out of the cell surface like a flagpole. People with blood types A, AB, and B have enzymes that modify the end of the flagpole, a process similar to raising a flag on the pole. In blood type O, no flag is raised. It's possible that some part of the process that raises the flag in blood type A and is missing in type O also performs some unknown function in ovarian cells, Pal said. "It is certainly possible" that these processes at work in blood type also at work in the ovary, "but there is little hard evidence to that effect," said Pamela Stanley, a professor of cell biology at Albert Einstein College of Medicine, who was not involved in the study Further understanding of the connection between blood type O and diminished ovarian reserve would be a good thing, Pal said. If the results of the study are replicated in healthy populations, women with blood type O might start having "the ticking clock conversation" in their early 20s, she said. Then, anyone deemed at risk for diminished ovarian reserve could avoid "complicating factors like being a smoker," she said. "Ovarian reserve is like a retirement account," Pal said. "You don’t worry about your retirement account only after you’ve retired. You need to keep an eye on your retirement account throughout your productive years." Pass it on: A woman's blood type and ability to conceive may be related, but lifestyle can also affect fertility.