Oestrogen

Oestrogen pills were used as contraceptives. According to Jack Harkness, by the 2000s, contraceptives could be identified in Earth's rain. As Jack explained, oestrogen entered the water cycle when people flushed their contraceptive pills down the toilet: sending the hormone "all the way up into the sky, [then] all the way back down on to me". Jack further claimed that this was "feminising" the fish. Nevertheless, he was happy enough to avoid another pregnancy. (TV: Everything Changes)