CHICAGO — If mom arrives at this year’s Mother’s Day gathering looking like she needs less stress and more relaxation, resist the urge to tell her to leave her job.Study after study shows that women who leave the workforce, whether it is to care for children or to retire early with a spouse, take a hefty financial hit.The price women pay for raising a family has been dubbed the “motherhood penalty,” with numerous academic studies quantifying the impact in thousands of dollars.Over a lifetime, mothers with one child earn 28 percent less than childless women, according to a recent study led by Matthew Rutledge for the Center for Retirement Research at Boston College.