Human chorionic somatomammotropin (HCS), is synthesize and secreted by the syncytiotrophoblast. Its effects on maternal metabolism are significant but the role of this hormone upon fetal development remains unknown. Nonetheless its measurement during final stages of pregnancy has proved to be useful for prediction of outcome. Since HCS serum levels exhibit progressive augmentation throughout gestation and taking into account its site of origin it has been proposed that could be dependent of changes in placental mass. This has not been totally ascertained, due to the lack of precision of studies designed for this purpose. If a correlation between HCS secretion and placental growth cold be established, it might be expected that determination of both indexes would contribute to obtain a more accurate diagnosis of abnormalities in retroplacental of fetal circulating blood volume. Therefore we studied 55 females without complicated pregnancy whose placental volumes were measured through ultrasound scanning. Serum samples were also collected for HCS quantitation by radioimmunoassay. Determinations were made starting on the 12th week of pregnancy. A progressive as well as a parallel increase of placental volume (154.12-825.01 ml) and HCS (0.48-7.0 Ug/ml) was observed during gestation. The correlation coefficient (r = 0.546) was significant (p less than 0.01). Both parameters correlated also with those obtained for fetal biparietal diameters. These findings support the notion that HCS secretion is proportional to the volume/mL of placental tissue which might be related to the amount of syncytiotrophoblast cells. It was not possible to establish causation upon the correlation observed between HCS and fetal parietal diameters.