pregnant (adj.2)
"convincing, weighty, pithy," late 14c., "cogent, convincing, compelling" (of evidence, an argument, etc.); sense of "full of meaning" is from c.1400. According to OED from Old French preignant, present participle of preindre "press, squeeze, stamp, crush," from earlier priembre, from Latin premere "to press" (see press (v.1)). But Watkins has it from Latin praehendere "to grasp, seize," and in Barnhart it is from Latin praegnans "with child," literally "before birth" and thus identical with pregnant (adj.1).
pregnant (adj.1)
"with child," early 15c., from Latin praegnantem (nominative praegnans, originally praegnas) "with child," literally "before birth," probably from prae- "before" (see pre-) + root of gnasci "be born" (see genus).

Retained its status as a taboo word until c.1950; modern euphemisms include anticipating, enceinte, expecting, in a family way, in a delicate (or interesting) condition. Old English terms included mid-bearne, literally "with child;" bearn-eaca, literally "child-adding" or "child-increasing;" and geacnod "increased." Among c.1800 slang terms for "pregnant" was poisoned (in reference to the swelling).