The secretion of placental isoferritin (PLF) during pregnancy could be responsible for the diminished immunoreaction of maternal lymphocytes against the embryo. In this study, serum levels of PLF in 335 pregnant women were correlated with birth weight, percentile of birth weight, gestational age, and pulsatility index of the uterine artery, as well as with diastolic notching. Women with small-for-gestational age (SGA) infants (birth weight below or equal to the tenth percentile) showed significantly lower PLF levels (11.7 U/ml) compared with women whose newborns were of normal weight (19.4 U/ml) (P < 0.05). In line with this finding, a strong correlation was observed between PLF levels and the percentile of birth weight, pulsatility index of the uterine artery, and the presence of diastolic notching. The strong correlation of low PLF values and SGA newborns suggests that determination of PLF could serve as a marker to identify women at risk of having such babies.