starstar-halfstar-outlinestar-outline(1.5 stars)

Vanessa Kirby delivers a bravura performance in “Pieces of a Woman.” In fact, her performance is so commanding, uncompromising and far-ranging that it often threatens to swallow this otherwise uneven and frustratingly thin movie with one voracious gulp.

Things get off to an optimistic start when Kirby's character, Martha, bids her office co-workers goodbye just before embarking on parental leave; her husband Sean (Shia LaBeouf) is doing the same thing at his job, working construction on a Boston bridge.

When Martha goes into labor in their cozy townhouse, they fly into action, calling the midwife and preparing for the blessed event. When a replacement named Eva (Molly Parker) arrives, they're taken aback but go with the flow: For the next
30 minutes, “Pieces of a Woman” chronicles the birth of Martha and Sean's daughter with startling intimacy, following the action in a fluid, unbroken shot that takes in every scintilla of pain, anticipation and relief of childbirth — which, in this case, involves deeply upsetting trauma.

It's a stunning piece of filmmaking, directed by Kornel Mundruczo with finesse and impressive logistical adroitness. This isn't a surprise considering his astonishing 2014 drama “White God,” which brought him to the attention of audiences outside his native Hungary. As in that film, Mundruczo evinces superb control with the camera, as well as an instinctive affinity for actors who can be seen committing entirely to even the most difficult material.

In this case, that subject matter is the aftermath of profound loss, which sends Martha and Sean into tailspins whose centrifugal force sends them further apart. After that opening sequence that veers so precariously on the edge of histrionics, Kirby turns on a dime, becoming preternaturally still, watchful and opaque. For his part, LaBeouf portrays Sean with a combination of self-pity and sympathetic pain, revealing once again what a gifted actor he is, up to and including his willingness to tap into his real-life battles with addiction.

With such impressive performances at its core, though, “Pieces of a Woman” begins to lose momentum and clarity. As Martha seeks to heal, her journey feels less and less convincing, especially her encounters with her brittle and combative mother (Ellen Burstyn). It's clear that Mundruczo has looked to the work of John Cassavetes and Ingmar Bergman for inspiration in his examination of how relationships break down amid grief, betrayal and isolation. But, even at their most raw and naturalistic, the scenes from Martha and Sean's marriage ultimately feel like a banal domestic melodrama, with more thespian pyrotechnics and icy, dead-eyed stares.

Even viewers who are captivated by the performances and technical prowess of “Pieces of a Woman” will be forgiven for finding its third-act twist both sentimental and wholly unearned. It also offers a tantalizing suggestion of a crucial relationship in the film that goes almost entirely ignored, or at least unplumbed — one that could have given “Pieces of a Woman” a genuinely novel and illuminating point of view. What we're left with is a showcase for Acting-with-a-capital-A, and little else.

R. At the Angelika Film Center Mosaic; also available on Netflix. Contains strong language, sexuality, graphic nudity and brief drug use. 126 minutes.