In pregnancy research, GrimAge is a measure of biological aging based on chemical markers on DNA, called DNA methylation. Researchers use it to study how stress and other factors may relate to aging.
A study of 77 pregnant women found that higher GrimAge acceleration was associated with shorter pregnancies and lower birth weights. However, this was a small study, and the findings do not prove that GrimAge causes those outcomes.
In plain English: GrimAge is an attempt to estimate how old the body appears biologically, rather than simply counting years of life.