After a still birth from her fourth pregnancy, for her fifth pregnancy, in chapter 17, O-Lan gives birth to twins. Wang-Lung's father declares, "An egg with a double yolk this time!" One is a boy and one is a girl.
It is after this birth, however, that O-Lan finally becomes barren, claiming she has a "fire" in her "vitals." She does not conceive again, and grows increasingly weaker, and sick. A doctor diagnoses her with "worms" in her heart and Wang Lung knows that whatever it is, it is terminal. She dies relatively young, just after seeing her eldest son get married.