Note: Zemach may have retained "Zemach, Margot" as her legal name after marriage but these sources do not say that:

• Margot Zemach Collection
• Wikipedia-EN
• LCCN

UMN.edu covers Fichstrom & Zemach as "the Zemachs" in prose; calls "Harve Zemach" a pseudonym for husband Harvey Fichstrom. (Perhaps LCCN would list "Fichstrom, Margot" as a variant only if it did appear in LC publication records or "Found in" sources; then this is no evidence at all.) LCCN does list "Fischtrom, Harvey" [sic] as a variant for him. [added by editor Pwendt 2015-12-18]: Her NYTimes obituary 1989-05-23 also spells her husband's surname "Fischtrom". It covers her as "Ms. Zemach" and closes with "four daughters, Heidi, Rachel, Rebecca and Kaethe Zemach-Bersin, and two granddaughters". Kaethe Zemach (professional name, perhaps sometime legal name) is a children's writer and illustrator under that name (c) 1979 and 2004. She wrote at least one book as by "Kaethe Zemach-Bersin", published 1988. Source LCCN