Sylvia Ann Hewlett first veered off the career highway - or "off-ramped", as she calls it - in her early 30s. She was assistant professor of economics at Barnard College, part of Columbia University, New York, and in the running for a permanent teaching position, when she lost twins in the seventh month of pregnancy. Then she lost her job too: she was told that she hadn't made tenure because she had "allowed childbearing to dilute [her] focus".