Something ancient awakens in our DNA during pregnancy and moments of blood loss to drive an increased demand for red cells in the body, researchers have found, reports ScienceAlert.

A new study by researchers in the US and Germany has uncovered long-dormant virus fragments triggering an immune response that ramps up red blood production when it's needed most.

The findings of the respective research were published in the scientific journal Science.

The surprising discovery was made through an analysis of hematopoietic (blood-forming) stem cells in mice, in which fragments of genetic code known as retrotransposons became activated during pregnancy.

Through an analysis of blood samples from pregnant and non-pregnant women, the researchers found it was likely that the same reactivation of retrotransposons seen in mice occurs in humans, too.

Further tests showed that when this process was blocked in mice, the animals developed anemia.

The team saw the retrotransposons were activating a signaling protein called interferon, which in turn increased hematopoietic stem cell activity.

Commonly referred to as “junk DNA,” retrotransposons were once dismissed as segments of code that appeared to be of little significance. It's now known that many of these genetic relics can still kick into action, sometimes even to our benefit.

As far as humans go, DNA passed down from viral infections in our ancestors make up around 8 percent of our whole genome, and these parts of the genetic code were once relegated to the background, which is why they were thought not to play a major role.