Blues striker Claudia Walker coolly fired her side in front with her fifth goal of a productive season to date but diverted into her own net at the far post when defending a set piece two minutes before half-time.
Walker then rattled the post early in the second half and despite both teams having plenty of moments of promise in an open and entertaining encounter, it was van Egmond’s 74th minute effort that proved decisive.
Blues led in the ninth minute with the in-form Walker timing her run in behind well to latch on to Mollie Green’s floated ball over the top before keeping her composure to tuck low to the right of keeper Mackenzie Arnold.
West Ham sought to hit back quickly, Martha Thomas driving into the box before backheeling to Alisha Lehmann but her shot from the corner of the six-yard box was saved by Hannah Hampton’s outstretched boot.
Sarah Mayling was inches away from doubling Blues lead, firing back across goal and against the base of the post when Green’s deep ball made its way to the right of the box and Jamie-Lee Napier had an even better chance to score on the rebound but placed wide.
Harriet Scott made a goal-saving intervention by covering a ball into the near post that Thomas seemed destined to convert from close range and Adriana Leon thumped over from the corner that followed.
The open nature of the game continued with Mayling’s cross finding Napier at the far post but the Chelsea loanee scuffed her finish.
Leon wanted a penalty having hit the deck after running at Scott but well-placed referee Richie Watkins was unmoved and the West Ham wide player was involved again with another wayward finish after latching on to Rebecca Holloway’s clearance from Gilly Flaherty’s header.
West Ham pressure had been building, though and the equaliser came two minutes before the interval when a short free-kick was played to Leon to cross to the far post and Walker diverted into her own net having tried to get back to mop up a flick-on.
Undeterred, Walker unleashed a 25-yard drive against the post with Arnold well beaten six minutes after the restart and openings kept coming at both ends only for the final ball or shot to be lacking amidst some last-ditch defending.
The decider came when full-back Rachel Daly floated into the box and van Egmond popped up level with the far post to tower over her marker and plant in from close range.
The game might have been up a minute later when substitute Kenza Dali took aim from 20 yards only to flash narrowly off target but the Hammers were visibly more confident from that point, snapping into tackles and managing the game and the time left in it effectively despite Blues’ best efforts.
Blues: Hampton, Scott, Brougham, Corsie, Holloway, Scofield, Green (Whipp, 65), Murray, Mayling (Kelly, 35), Walker, Napier (Lawley, 87).
Unused subs: McCarron, Toussaint.
Goal: Walker 9.
West Ham: Arnold, Daly, Flaherty, Fisk, Vetterlein, Cho, van Egmond, Lehmann (Kiernan, 64), Svitkova (Dali, 52), Leon, Thomas (Longhurst, 80).
Unused subs: Brosnan, Redisch, Joel, Cissoko, Grant, Nor.
Goals: Walker (og) 43, van Egmond 74.
Yellow cards: Vetterlein 31, Daly 90+3.
Referee: Richie Watkins.