PIP: It was hypothesized that prolactin might be involved in the mechanism of twinning along with the pituitary hormones, namely, follicle stimulating hormone (FSH), luteinizing hormone (LH), hypothalamic gonadotropin releasing hormone (GnRH), and gonadotropin hormone (GH). It is known that lactation prolactin induces a gonadotropin mediated amenorrhea. Otherwise, weaning results in low serum prolactin and high FSH and LH secretion leading to the resumption of ovulation. Variations in serum prolactin in women carriers of twin gene may have a bearing on their gonadotropin mediated twinning liability. The hypothesis was tested using data from a small rural population characterized by a shift from total breastfeeding to partial breastfeeding with an early introduction of solid foods to infants. It is reported that the changing pattern in the practice of breastfeeding in the community between 1800 and 1939 led to an increased incidence of monozygotic (MZ) twinning. This study of twin births was carried out from the data obtained through parish registries. From colonization to 1967, families were reconstructed through computer programs linking certificates of births, deaths, and marriages. Age standardized twinning rates were computed for the last 2 studied time periods by the indirect method, taking the 1800-1869 time period as reference. The overall incidence rate of twinning seems to have increased over the commonly cited rate of 1.2% for whites in the course of the 3 time periods, i.e., 1800-1869, 1870-1899, and 1900-1939. The trend was in contrast to the overall standardized twinning rates which, based on the changing fertility patterns of the population, point to an expected lowering of twin births. The classical Weinberg formula allows allocation of the twins to the MZ and dizygotic (DZ) classes. The data suggest an increase in MZ twins over time that may not be tested statistically. Yet, it may be noted that if a confidence interval of 2 standard errors about the total MZ twinning rate is taken, the 1800-1869 and 1900-1939 MZ twinning rates are nearly excluded, thus pointing to a possible increase over time. The frequency of DZ twins did not change during the time interval. Because the twinning rate is positively correlated with the population fertility pattern of this rural area, it is plausible to link twinning to fertility for biological reasons, the more so since twin bearing mothers are thought to be more fecund. It is possible that lactation prolactin, because of its relation to FSH and LH secretion, may be the missing link in the hormonal diathesis.