When does the 'first trimester' start, in this regard? Modern medicine counts the start from the first day of the last period before a woman conceived. (Which seems weird - saying someone's pregnant during a time when they were on their period. Any doctors who want to elaborate?) If you count one trimester from that date, you get a date about two weeks earlier than if you count from the mikveh night following the last period.
Also, is 'one trimester' 12 weeks? 90 days? From the date a woman becomes pregnant (e.g. 15 Nisan) to the same date three calendar months later? Something else?
The last Lubavitcher rebbe quoted in many personal correspondences, in the name of his father in-law, that a woman should not reveal her pregnancy until the fifth month, except to her doctors, and it is also fine to reveal to relatives or close friends. This tradition was incorporated into the halachic work of the last rebbe's teachings too (Shaarei Halachah U'minhag, EH no. 8).
In the book Torat Ha-yoledet from Rabbis Zilberstein and Rothchild (pub. 1983, p. 304) this same instruction is stated without mention of any exclusion.