Bilirubin can be detected in normal amniotic fluid after about 12 weeks of gestation, but it disappears by 36 to 37 weeks’ gestation. The ability of human fetal liver to remove bilirubin from the circulation and to conjugate it is severely limited. Between 17 and 30 weeks of gestation, uridine diphosphoglucuronosyl transferase (UDPGT) activity in fetal liver is only 0.1% of adult values, but it increases tenfold to 1% of adult values between 30 and 40 weeks’ gestation. After birth, activity increases exponentially, reaching adult levels by 6 to 14 weeks’ gestation (Fig. 35-4). This increase is independent of gestation (Table 35-1) (12,13).