ESR1 and cancer: Another study indicated that every childbirth reduces the risk of progesterone receptor (PR+) and estrogen receptor (ER+) cancers by up to 10% (risk ratio (RR) per birth, 0.89; 95% CI, 0.84-0.94), and women who were older at their first childbirth had a 27% increased risk of developing breast cancer (RR, 1.27; 95% CI, 1.07-1.50) [37,38].