Texas mother's sets of identical twins

Tressa Montalvo places her hand on her newborn son, Blaine, at The Women's Hospital of Texas on Feb. 18, 2013, in Houston. Montalvo gave birth to two sets of identical twins on Valentine's Day, born at 31 weeks. The arrival of Ace and Blaine then Cash and Dylan a minute later was even more unusual because the Montalvos did not take fertility drugs or use in-vitro fertilization.

Credit: AP Photo/Houston Chronicle/ Brett Coomer