The article is based on correspondence between Margaret Sanger, an American activist of the birth control movement, and Dr. Herman Rubinraut, a Polish physician involved in work focused on conscious motherhood. Heretofore unused American sources made it possible to expand our knowledge about the Polish birth control campaign during the interwar period and in the People's Republic of Poland. At the same time, they have provided an opportunity for following the almost thirty–years long unofficial contacts between the two social reformers. The examined correspondence deals with professional topics and the efforts made by Dr. Rubinraut (who was of Jewish descent) to emigrate in view of the looming Nazi threat. The sources make it possible to resolve the question of mutual impact and inspiration, involving Polish and American organisations, and to notice the similarity of the problems and challenges tackled by activists on both sides of the Atlantic.