Kvaterin:[1] [Achronim]

A pregnant woman should not be a Kvaterin. However, some Poskim[2] rule that this only applies if her pregnancy is recognizable.

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[1] Bris Avos 5:26; Likkutei Sichos 22:56-58; Sefer Shemiras Haguf Vihanefesh [Lerner] 152:1; See Shevach Habris 1:17 and footnote 29

[2] Minchas Elazar on Sefer Zocher Habris; Sefer Shemiras Haguf Vihanefesh [Lerner] 152:2