"with child, impregnated, that has conceived in the womb," 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" (from PIE root *gene- "give birth, beget").
The word tended to be avoided in polite conversation 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).
["convincing, weighty, pithy, full of meaning"] late 14c., "cogent, convincing, compelling" (of evidence, an argument, etc.); c. 1400 as "full of meaning;" from Old French preignant "pregnant, pithy, ready capable," which is probably from Latin praegnans "with child, pregnant, full" and thus the same word as pregnant (adj.1).
All uses seem to be derivable from the sense of "with child." But in some sources this English pregnant has been referred to French prenant, present participle of prendre "to take," or to the French present participle of preindre "press, squeeze, stamp, crush," from earlier priembre, from Latin premere "to press, hold fast, cover, crowd, compress." The two English adjectives are so confused as to be practically one word, if they were not always so.