The Leboyer birth method requires that the newly born infant is placed on the mother's abdomen and the cord is clamped when it stops pulsating. This investigation was done to study the effect of Leboyer childbirth on neonatal circulation during the first 5 days after birth. Hematocrit, blood viscosity, left and right ventricular output, and cerebral blood flow velocities in the arteria carotis interna, arteria cerebri anterior, and truncus coeliacus were studied in 15 full-term neonates with early (less than 10 seconds) cord clamping and 15 full-term neonates delivered according to Leboyer (cord clamping after 3 minutes) on day 1 (2 to 4 hours after birth) and day 5. The fetal placental blood volume decreased from 42 +/- 8 mL/kg (mean +/- SD) of neonatal body weight after early cord clamping to 19 +/- 7 mL/kg after Leboyer delivery. Neonatal blood volume, calculated from the fetal placental blood volume, was 32% higher in the Leboyer group compared with the early cord-clamped infants. In the infants with early cord clamping, hematocrit, and blood viscosity did not change significantly during the first 5 days. After Leboyer birth, the hematocrit rose from 0.51 +/- 0.05 in cord blood to 0.62 +/- 0.06 at 2 to 4 hours of age, thereby increasing blood viscosity by 32%. Stroke volume, heart rate, cardiac output, left-to-right shunt across the ductus arteriosus, and blood flow velocity in the truncus coeliacus were similar in both groups and did not change during the first 5 days.(ABSTRACT TRUNCATED AT 250 WORDS)