Massive decline in lung liquid before vaginal delivery at term in the fetal lamb

Objective: Our aim was to determine the volume of liquid remaining in the lungs of the fetal lamb just before a normal vaginal delivery at term to assess the extent to which an excess of liquid in the airspaces might contribute to the respiratory morbidity that accompanies elective cesarean delivery.

Study Design: The volume of liquid in the future airspace of the lungs was determined at the end of labor in eight fetal lambs at term from the dilution of an impermeable tracer (¹²⁵I-labeled human serum albumin) mixed into the liquid. This volume was compared with that measured in a second group of 10 fetal lambs studied 7 days before the expected date of delivery (term=147 days).

Results: The volume of lung liquid present at the end of labor was 6.8 ± 1.0 ml · kg⁻¹ (n=8) compared with 28.2±1.8 ml · kg⁻¹ (n=10) in the second group of lambs studied before the onset of labor at 140 days of gestation.

Conclusion: Our results indicate that the bulk (>75%) of the liquid that fills the lungs of the fetal lamb at 140 days of gestation is cleared at some time before normal term birth, suggesting that the adverse respiratory impact of elective cesarean delivery may be largely explained by denying the fetus this important adaptive mechanism.