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Unlabelled: Our objective was to determine arterial oxygen saturation as measured by pulse oximetry (SpO2) in healthy term neonates during their first 4 weeks of life. Overnight recordings of SpO2 (Nellcor N200), photoplethysmographic (pulse) waveforms from the oximeter and breathing movements were performed in 60 term infants. They were studied initially during their 1st week of life (median age 4 days, range 1-7) and then again during their 2nd-4th week (median age 17 days, range 8-27). Median baseline SpO2, measured during regular breathing, was 97.6% (range 92-100) during week 1 versus 98.0% (86.6-100) during week 2-4 (P > 0.05). Episodes of desaturation, defined as a fall in SpO2 to < or = 80% for > or = 4 s, were found in 35% of recordings obtained in week 1 compared to 60% of those obtained in week 2-4 (P < 0.01). Their frequency increased from a median of (0-41) per 12 h of recording at the initial recording to 1 (0-165) at follow up (P < 0.01). Analysis of the data by week of life showed a peak in desaturation frequency in the 2nd week of life. The infants with extreme values at follow-up (e.g. a baseline SpO2 of 86.6%, 5th percentile 91.9%, or a desaturation frequency of 165 per 12 h of recording, 95th percentile 32) had had values well within the normal range during their initial recording (a baseline SpO2 of 94.4%, or a desaturation frequency of 4). Most of the desaturations in the infants with extreme values were associated with periodic apnoea. These results demonstrate only relatively minor developmental changes in oxygenation in term neonates during the first 4 weeks of life. The clinical significance of outlying values, i.e. a low baseline SpO2 or a high number of episodic desaturations, remains to be determined., Conclusion: These healthy term neonates had values for baseline oxygen saturation and desaturation frequency that were not substantially different from those observed in older infants.