A relationship between neonatal breast size and cord blood testosterone level.

The present study was performed to determine if any hormone measured in cord blood correlates with the size of the neonatal breast or the presence of galactorrhea. A total of 144 term newborn infants were examined. Estradiol (E2), progesterone (P), testosterone (T), and thyrotropin (TSH) were determined by radioimmunoassay (RIA), and prolactin (PRL) was determined by both RIA and biological activity (BA). The female breast (8.5 +/- 2.0 mm) was found to be larger than that of the male (7.8 +/- 2.1 mm, p less than 0.05). The only hormonal difference between sexes was a higher T level in the male infants (8.0 +/- 3.0 nmol per L vs. 5.5 +/- 1.9 nmol per L, p = 0.002). None of the other hormones measured by RIA correlated with the size of the neonatal breast or the presence of galactorrhea. The BA of PRL was widely variable compared to the PRL RIA but also failed to correlate with neonatal breast size or galactorrhea. This study suggests that T might be one factor in determining the size of the neonatal breast.