['To examine the effect of time after birth on heart rate (HR) measured by pulse oximetry (PO) (HRPO) and electrocardiography (ECG) (HRECG).', 'HRECG and HRPO (collected at maximum sensitivity) were assessed in 53 term and preterm infants at birth. ECG electrodes and a PO sensor were attached as soon as possible and HRECG and HRPO were compared every 30 seconds from 1-10 minutes after birth. Data were compared using a Wilkinson signed-rank test. Clinical relevance (eg, HR <100 beats per minute [bpm] was tested using a McNemar test).', 'Seven hundred fifty-five data pairs were analyzed. Median (IQR) gestational age was 37 (31-39) weeks. Mean (SD) starting time of PO and ECG data collection was 99 (33) vs 82 (26) seconds after birth (P = .001). In the first 2 minutes after birth, HRPO was significantly lower compared with HRECG (94 (67-144) vs 150 (91-153) bpm at 60 seconds (P < .05), 81 (60-109) vs 148 (83-170) bpm at 90 seconds (P < .001) and 83 (67-145) vs 158 (119-176) at 120 seconds (P < .001). A HR <100 bpm was more frequently observed with a PO than ECG in the first 2 minutes (64% vs 27% at 60 seconds (P = .05), 56% vs 26% at 90 seconds (P < .05) and 53% vs 21% at 120 seconds (P < .05). HR by ECG was verified by ultrasound for outflow from a subset of infants.'] Does pulse oximetry measure a lower heart rate at birth compared with electrocardiography?