Women are more sensitive than men to a number of smells. The female hormone estrogen may account for this sensory difference. As estrogen levels rise and fall during a monthly cycle, her sensitivity to odors rises and falls.
Women are most sensitive to smell during ovulation, when estrogen levels are highest, and less sensitive during menstruation, when estrogen is lower. The estrogen levels of pregnant women drop drastically. It is estimated that a woman is 2,000 times more sensitive to smell before her pregnancy than during it.
The "pickles-and-ice cream" syndrome, in which pregnant women eat unusual combinations of food, is thought to be due at least partly to a culled sense of smell during pregnancy. Hence pregnant women can tolerates that they found unappetizing before becoming pregnant.