S. Rugpao, K. Amatayakul, Y. Yutabootr, S. Suwankiti, A. Sontirat, and S. Sethasathien, “Vaginal Bleeding Patterns Among Lactating Women Using Contraceptive Methods”, Thai J Obstet Gynaecol, vol. 4, no. 1, pp. 33–41, Jan. 1992.