WordNet records no synonym and no antonym for placenta previa. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
pregnancy in which the placenta is implanted in the lower part of the uterus (instead of the upper part); can cause bleeding late in pregnancy; delivery by cesarean section may be necessary
Synonyms, antonyms and the broader and narrower relations for placenta previa come from Princeton University’s WordNet. Full sources and licences.