Effect of progesterone on the initiation of respiration of the newborn.

On the 30th day of pregnancy, the experimental fetuses in 19 rabbits were injected intraperitoneally with 0.1-1.0 mg progesterone (P). The control fetuses in the contralateral uterine horn were treated with the solvent, sesame oil. Both the experimental and control fetuses were delivered 10 min after treatment by hysterotomy to measure the P concentration of their lung tissue and observe their respiration during the initial 2 h of postnatal life. In comparison with the controls, the experimental newborns had greatly elevated P levels and showed typical respiratory distress syndrome, confirmed by histology.