Prolactin, maternal oestrogens, and breast development in the newborn.

To investigate the influence of maternal oestrogens on the fetal breast development, maternal urinary oestriol excretion, maternal plasma oestriol concentrations, and cord venous plasma oestradiol and oestriol concentrations were related to the size of the neonatal breast. A significant positive association between oestriol excretion and neonatal breast size was demonstrated, but the relationship was not strong and might be due to both measures having a positive relationship with birthweight. The infants' circulating concentrations of prolactin at birth and during the first weeks of life were also related to breast size. There was no cord venous-arterial difference in prolactin concentrations, and neither related to breast diameter. However there was a strong association between breast size and prolactin concentrations in mature infants aged between 5 and 7 days. In preterm infants breast tissue often develops after birth. Prolactin levels in preterm infants were higher between 2 and 6 weeks than they were in the first week of life. It would appear that the early development of the breast is influenced more by the infants's than the mother's endocrine activity.