Respiration Physiology (1986) 65, 55-68 55 Elsevier PHASIC RESPIRATORY ACTIVITY IN THE FETAL LAMB DURING LATE GESTATION AND LABOUR P.J. BERGER, A.M. WALKER, R. HORNE, V. BRODECKY, M.H. WILKINSON, F. WILSON and J.E. MALONEY Monash University Centre for Early Human Development, Queen Victoria Medical Centre, 172, Lonsdale Street, Melbourne, Victoria 3000, Australia Abstract. We quantified the respiratory activity of 9 fetal lambs using computer-analysis of the diaphrag- matic electromyogram (EMG) obtained during 2 h recording sessions interspersed over the last 13 days of gestation. The fetuses delivered unassisted at an average gestational age of 145 days (term = 147 days). During the last 2 h of labour the number of phasic EMG bursts (breaths) averaged 3 ~o of the peak recorded earlier in the study. This decline in breathing began at least 2 days before labour and resulted predominantly from the fetus spending an increasing proportion of time in apnoca. Respiratory rate within epochs of breathing also fell significantly 1 day before labour, and the proportion of time spent in the low voltage electrocortical state declined once labour commenced. No significant change occurred in arterial Po2, P¢o2 or pH over the study period. We conclude that fetal respiratory activity falls well before the onset of labour, largely as a result of increased apnoea, and that the decline does not result from the development of a progressive hypoxaemia associated with labour. Arterial blood Diaphragm Gestation Lamb Breathing pattern Fetus Labour Newborn mammal Breathing has been recorded in fetal lambs from as early as 40 days gestation (Dawes et al., 1972). From that age up to about 120 days, when the electrocortical activity becomes differentiated into low voltage (LV) and high voltage (HV) states, breathing is virtually continuous (Bowes etal., 1981); thereafter phasic respiratory activity is restricted almost entirely to the LV state (Dawes et al., 1972). Although the respiratory activity of fetal lambs older than 120 days has been studied many times, respiratory activity in the days leading up to labour has not been examined adequately. Dawes and colleagues reported on 7 fetuses during labour; 3 were in labour 'at or near term' (Dawes et al., I972) and 4 were in premature labour (Boddy et al., 1974). The authors concluded that fetal breathing continued unaltered until the onset Accepted for publication 29 March 1986 0034-5687/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)