A 'viable fetus' is a fetus 'so far formed and developed that if then born it would be capable of living.' Commonwealth v. Crawford, 430 Mass. 683, 689, 722 N.E.2d 960 (2000), quoting Torigian v. Watertown News Co., 352 Mass. 446, 448, 225 N.E.2d 926 (1967).A viable fetus also is a 'person' for purposes of our motor vehicle homicide statute, G. L. c. 90, § 24G. See Commonwealth v. Cass, 392 Mass. 799, 806, 467 N.E.2d 1324 (1984) (rejecting common-law rule that person must be 'born alive' to be protected by criminal law, stating 'medical science now may provide competent proof as to whether the fetus was alive at the time of a defendant's conduct and whether his conduct was the cause of death').