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Contraception. 1984 Dec; 30(6):505-22.The World Health Organization (WHO) conducted a randomized comparative trail of th effects of hormonal contrception on milk volume and infant growth. The 341 study participants, drawn from 3 obstetric centers in Hungary and Thailand, were 20-35 years of age with 2-4 live births and previous successful experience with breastfeeding. Subjects who chose oral contraception (OC) were randomly allocated to a combined preparation containing 150 mcg levonorgestrel and 30 mcg ethinyl estradiol (N=86) or to a progestin-only minipill containing 75 mcg dl-norgestrel (N=8). 59 Thai women receiving 150 mg depot medroxyprogesterone (DPMA) intramuscularly every 3 months were also studied. An additional 111 women who were using nonhormonal methods of contraception or no contraception served as controls. Milk volume was determined by breast pump expression. No significant differences in average milk volume were noted between treatment groups at the 6 week baseline visit. However, between the 6th and 24th weeks, average milk volume in the combined OC group declined by 41.9%, which was significantly greater than the declines of 12.0% noted in the progestin-only group, 6.1% among DMPA users, and 16.7% among controls. The lower expressed milk volume among combined OC users did not impair infant growth. No significant differences were observed between treatment groups in terms of average infant body weight or rate or weight gain. Users of combined OCs may have compensated for their decreased milk volume by providing more extensive supplementary feeding or more prolonged suckling episodes. These results suggest that the estrogen content of combined OCs adversely affects the capacity of the breast to produce milk; thus, family planning programs should make nonestrogen-containing methods available to breastfeeding mothers. Although no effects on infant growth were noted in this study, the possibility of such efects cannot be excluded in populations where infant growth largely depends on the adequacy of unsupplemente d lactation.