Fetal breathing movements have been observed to be present in the human fetus from as early as 12 weeks of gestation. These movements have been seen primarily in rapid-eye-movement (REM) sleep. They are stimulated by hypercapnia, acidemia, pilocarpine, ACTH, and steroids, and are depressed by hypocapnia, alkalemia, hypoxia, and maternal smoking. The seemingly paradoxical response of the fetus to hypoxia is similar but not identical to the steady-state response to hypoxia of the term newborn infant, suggesting a continuum of response from fetal to postnatal life.
T. B. Waggener (Wed,) studied this question.