Warning: Wikipedia contains spoilers.
Keiko married Miles O'Brien aboard the USS Enterprise in the TNG episode "Data's Day". A year later, on Ten-Forward, she gave birth to a daughter, Molly, with Worf as midhusband (TNG episode "Disaster"). When Miles was reassigned to Deep Space Nine, Keiko decided to start a school, and Jake Sisko and Nog were the first students to enroll. Later on Keiko went on an archeobotanical expedition to Bajor. After becoming pregnant with her second child, Keiko returned to Deep Space Nine. Unfortunately, an accident endangered mother and child. Doctor Julian Bashir saved both by removing the fetus and implanting said fetus into Kira Nerys's womb. In her honor, the child was named Kirayoshi O'Brien.