Human versus animal insulin in the management of insulin-dependent diabetes: Lack of effect on fetal growth

It is generally accepted that the human placenta is impermeable to free insulin and that insulin present in the fetus is entirely of fetal origin. A recent study suggested that antibody- bound animal insulin crosses the placental barrier and may exert direct effects on fetal growth. We hypothesized that mothers with insulin-dependent diabetes treated with animal insulin would have infants with higher birth weights and ponderal indices compared with mothers treated with human insulin. We studied 209 mothers with insulin-dependent diabetes who were enrolled in our program and who delivered after 28 weeks’ gestation: 170 were treated with animal insulin and 39 with human insulin. There were no differences between the groups in the mean birth weight (adjusted by gestational age at delivery) or ponderal index of the infants. The rate of macrosomia (birth weight greater than the 90th percentile for gestational age or ponderal index above 2.85) was similar in both groups. The sample size was adequate to yield a power of 80% to detect a difference between groups of 179 g or more in birth weight and 0.1 g/cm³ in ponderal index. We suggest that the type of insulin (animal versus human) used by the pregnant insulin- dependent diabetic mother has no bearing on fetal weight gain.