The days when advanced degrees went hand in hand with childlessness appear to be disappearing. A study released Thursday by the Pew Research Center finds that just 20 percent of women with a Ph.D. or an M.D. are remaining childless, down from 35 percent in 1994. Pew was unable to provide a figure for Ph.D. recipients alone. The study, based on U.S. Census data, confirms that overall the societal trends are toward smaller American families, but finds that the gap between highly educated women and others is closing. About 6 in 10 women with at least a master’s degree had two or more children in 2014, up from 51 percent in 1994.