Bouts of hiccuping are recognized by pregnant women as distinct episodic movements of their fetuses. Ultrasound imaging of these fetuses has documented the occurrence of hiccups from early gestation through parturition. This study provides a systematic characterization of prenatal hiccuping in the fetal baboon (Papio species). Tracheal fluid pressure was recorded from 11 chronically instrumented fetal baboons for 21.5 +/- 7.3 consecutive days (mean +/- SD) over a range in gestation from 124 to 164 days (term 175 days). In an initial review of pressure recordings by visual inspection, hiccups were recognized as distinctive high-amplitude fluctuations in tracheal pressure that were readily discriminated from fetal breaths. Automated techniques were then developed and validated to detect hiccups and summarize their features. The mean hiccup amplitude was 23.0 +/- 3.1 mmHg, inspiratory time was 0.26 +/- 0.03 s, and expiratory time was 0.27 +/- 0.02 s. Each of these features discriminated hiccups from breaths (P < 0.001). Hiccuping incidence (1.8 +/- 0.4% of time), rate (26.2 +/- 6.2 min-1), bout duration (4.3 +/- 0.8 min), and the interval between bouts (3.35 +/- 0.60 h) were also different (P < 0.01) from breathing. These features of hiccups remained relatively constant over the latter third of gestation with the exception of an increase in duration of the expiratory time interval (r = 0.54, P < 0.01). Despite their vigorous nature, bouts of hiccuping were not associated with transitions in behavioral state. Moreover, the features of hiccups were not differentiated by state. Bouts of hiccuping recurred in a cyclic fashion, on average every 3-4 h.(ABSTRACT TRUNCATED AT 250 WORDS)