Expulsion of liquid from the fetal lung during labour in sheep

Effective gas exchange after birth requires clearance of most of the liquid filling the lung during gestation. To date the focus has been on active Na+ transport from lung lumen to interstitium, but Na+ transport begins only close to delivery, making it an unlikely mechanism for clearing the bulk of fetal lung liquid. We hypothesised that fetal trunk muscle contractions, known to occur in labour, are involved in lung liquid clearance. We measured maternal uterine contractions, fetal tracheal flow directly and fetal electromyograms in thoracic and abdominal muscles. During labour in five fetal sheep, brief flow pulses were observed in the trachea, most of which expelled a small volume of lung liquid. Tracheal flow pulses were associated with fetal muscle contractions 89% of the time, which were associated on 91% of occasions with uterine contractions. Our results suggest that liquid contained in the fetal lung is cleared before and during labour as a result of fetal muscular effort, perhaps stimulated by uterine contractions.