The 'official line' is that his first wife, Lady Florence died during childbirth
4 April 1873
'John Henry de la Poer Beresford, fifth Marquis was married twice, first to Lady Florence Grosvenor (who died in childbirth and whose monument is inserted in a window at Clonegam, Waterford) and later Lady Blanche Somerset, only, daughter of the 8th Duke of Beaufort'.