If a female tennis athlete just learn she is pregnant. when does she has to stop training druing pregnancy? is it during the 2nd trimester?

Around 20 weeks: There is no one right answer to this question. A general approach for an athlete who has been training vigorously (including high-intensity workouts) is to continue training as able through the first half of pregnancy, and during the second half of pregnancy to scale things back to a "maintenance" mode. All of this depends also on the individual mother, number of fetuses, and pregnancy course.