פירוש על עבודה זרה 117:7

Yohanan holds that until a convert circumcises and immerses he is considered a non-Jew. So these women were pregnant from non-Jews. And R. Yohanan also holds that if a non-Jewish man impregnates a Jewish woman, the offspring is a mamzer. So these children are mamzerim. Note that this is not the accepted halakhah.