Are there perils in giving a pay raise in hopes of boosting employee performance?

Q. About four months ago, we gave a pay raise to a marginal employee who is pregnant, in hopes that it would improve her job performance by boosting her morale. Unfortunately, her performance has gone from bad to worse. If we fire her for poor performance, can she successfully argue that the recent raise indicates that she was performing well and that our reason for terminating her was discriminatory?