Why women with blood type A have a lower risk of preterm birth
Women with blood type A may be better protected against certain infections. Why? Because their vagina contains higher levels of a beneficial bacterium called Lactobacillus crispatus.
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About this article
More than one in ten children worldwide are born
(sidenote:
Preterm birth
birth before 37 completed weeks of gestation. There are sub-categories of preterm birth, based on gestational age:
- extremely preterm (less than 28 weeks);
- very preterm (between 28 and 32 weeks);
- moderate to late preterm (between 32 and 37 weeks).
Source: WHO
)
. A British team recently analyzed data from thousands of young mothers 1 to show that blood type influences the composition of the maternal vaginal microbiota, and with it, immunity and pregnancy outcomes: women with blood type A may be better protected against certain infectious risks thanks to the increased presence in the vagina of beneficial bacterium, Lactobacillus crispatus.
How is this possible?
You’re probably aware that the four main blood types are A, B, AB, and O. These letters indicate the presence of large sugar molecules called antigens, of type A and/or B (or their absence in the case of type O), on the surface of red blood cells. Among other things, these groups determine compatibility for transfusions.
What is less well known is that these “marker” antigens are not only found in the blood: they are also present in other cells (e.g., in the vagina or cervix) and in bodily fluids (including those produced by the cervix and vagina). By contributing to the adhesion or feeding of bacteria, these antigens influence susceptibility to infections. For example, people with type O blood are statistically more vulnerable to Helicobacter pylori, while those with type B blood are more vulnerable to bacteria such as E. coli.
Does each blood type have its own vaginal flora?
Researchers have now confirmed that A, B, and O antigens play a role in vaginal microbiota composition. Women with blood type A are more likely to have a vaginal microbiota dominated by L. crispatus, a beneficial bacterium that very easily attaches to the A marker. A direct consequence of this connection is a less inflammatory vaginal flora and a higher chance of full-term delivery.
Conversely, L. crispatus is often depleted in women with blood type O, particularly high-risk women due to a previous preterm delivery, and those with blood type B, who generally harbor more of the pathogen S. agalactiae, a bacterium that easily attaches to the B marker.
Some figures on premature birth
- An estimated 13.4 million babies were born preterm in 2020 (before 37 completed weeks of gestation) ².
- Across countries, the rate of preterm birth ranges from 4–16% of babies born in 2020 ².
- More than 90% of extremely preterm babies (less than 28 weeks) born in low-income countries die within the first few days of life, yet less than 10% of extremely preterm babies die in high-income settings ².
- Preterm birth complications are the leading cause of death among children under 5 years of age, responsible for approximately 900 000 deaths in 2019 ².
- Three-quarters of these deaths could be prevented with current, cost-effective interventions ².
Not so fast
However, do not over-interpret these results, or take liberties with your vaginal flora if you’re in group A. The effect of blood type on the risk of preterm birth remains low and is much less important than factors such as ethnicity (African and Asian women are at higher risk) or medical history. This discovery should primarily be seen as a gateway to future antigen-based therapies aimed at preventing preterm births.
