Natera Inc. /zigman2/quotes/208435998/composite NTRA -0.75% shares fell in the extended session Tuesday after the DNA test company announced a secondary offering of its stock. Natera shares fell 6% after hours, following a 2.7% gain to close the regular session at $39.22. The company said it plans to offer $175 million in shares, with underwriters getting an option for $26.3 million to cover overallotments. J.P. Morgan, Morgan Stanley, Cowen, and Piper Jaffray are among the underwriters. At Tuesday's closing price, about 5.1 million shares will be offered, compared with about 70 million shares currently outstanding.