Morning sickness was a symptom of pregnancy. It manifested as nausea in Humans and sneezing in Bajorans.
In 2372, Jadzia Dax and Miles O'Brien were discussing Keiko O'Brien's pregnancy. O'Brien expressed his dismay at his wife's risky behavior, saying it was as though he had to remind her she was pregnant, to which Jadzia replied that all the symptoms of pregnancy, including morning sickness, weren't enough of a reminder.
Later, Keiko and Kira Nerys discussed the differences between women of each of their species while Kira was carrying Keiko and Miles O'Brien's son. Kira speculated that it might be different because Keiko's child was a Human baby. (DS9: "Body Parts")
In 2375, Benjamin Sisko reminded a pregnant Kasidy Yates that morning sickness ended after the first trimester. (DS9: "What You Leave Behind")
In a deleted scene from TOS: "Who Mourns for Adonais?", Carolyn Palamas, who is revealed by Leonard McCoy to be pregnant with Apollo's baby, is said to have become ill at breakfast. (Star Trek: Lost Scenes)