Women's greater longevity may partly be linked to stem cells acquired during pregnancy, according to a team from Hammersmith Hospital and Imperial College London.[1] Fetal stem cells acquired during the first trimester and harbored in the bone marrow wait to migrate to sites of tissue injury or disease, says Nicholas Fisk, professor of reproductive and developmental biology at Imperial College. "The numbers [of fetal stem cells] we found were very small, [one in 70,000 to one in 450,000], but these may still be significant." The researchers used fluorescent in situ hybridization probing for Y chromosomes in women who had birthed male children as many as 50 years earlier.