Type 'Immaculate Conception' into a Topic Guide. The only suggestion is 'Virgin Birth of Christ'. Not quite the same thing... At least not for those who are likely to search for that topic.
The real irony comes when one runs that 'Virgin Birth of Christ' Guide and looks in the Definitions section. After a number of 'Virgin Birth' entries, the last one immediately visible for me is "IMMACULATE CONCEPTION, THE" from the 1915 ISBE. Clicking on that link, I can then read:
"IMMACULATE CONCEPTION, THE
<i-mak’-u-lat kon-sep’-shun>:
DEFINITION:
The historic designation of the Roman Catholic dogma promulgated by Pope Pius IX on December 8, 1854, in the Papal Bull entitled “Ineffabilis Deus.” The term is often incorrectly applied, even by those whose intelligence should make such an error impossible, to the VIRGIN BIRTH OF CHRIST (which see)."
I guess even that rather... outspoken warning wasn't enough to prevent you from tagging the article to precisely 'Virgin Birth of Christ'...
How long will it take before we finally get (correctly tagged!) Catholic (and Orthodox) topics? It's long overdue!