Recapped.com has posted a review of the nude scenes in Lars Von Trier's Nymphomaniac: Volume I. To paraphrase, Stacy Martin shows everything (including one scene with a brief view of labia). Any explicit penetration scenes are done by a double. Sophie Kennedy Clark (Philomena) briefly also shows breasts in Chapter 1 as she is changing clothes. There is no nudity from Charlotte Gainsbourg in Volume I. The full text of the review is below.
http://www.recapped.com/nymphomaniac-volume-1-nudity-review#comments
Nymphomaniac: Volume 1 premieres on-demand March 6 and in theaters on March 21. Volume II premieres on-demand April 3 and in theatres on April 18.
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Here is a link to the red-band trailer from Lars Von Trier's new film Nymphomaniac. This trailer features quite a bit explicit sex and nudity, especially from Stacy Martin (playing the younger version of Charlotte Gainsbourg's character.