1. Kvaterin by Bris and Shushvinin by wedding:^[1]

Bris Mila: It is customary in many communities[2] for a pregnant woman not to be a Kvaterin by a Bris Mila.[3] However, some Poskim^[4] rule that this only applies if her pregnancy is recognizable.

Wedding-Shushvinim; Unterefririn:[5] It is customary in many communities for a pregnant woman not to be the escort of the bride or groom, known as the Unterefririn, by a wedding. This is due to the same reasons as for why a pregnant woman should not be the Kevatrin by a Bris. There is no clear Chabad custom in this matter, and hence the Rebbe’s final directive in this matter is as follows:[6] If the mother of the groom or bride is pregnant, then another couple is to join them [the father and pregnant mother of the groom or bride] in escorting the groom or bride and in encircling the bride, and in God’s eyes it is known who is the true escort. However, to instruct them not to escort the bride or groom because she is pregnant would be a great source of pain for the parents, and hence she should do as above.

[1] Bris Avos 5:26 in name of Likkutei Sefer; Toras Alef 3:34; Zocher Habris 11:13; Michtav Oz of Minchas Elazar; Likkutei Sichos 22:56-58 [custom of many places], printed in Shaareiy Halacha Uminhag 2:34 and Shulchan Menachem 5:111; Sefer Shemiras Haguf Vihanefesh [Lerner] 152:1; See Shevach Habris 1:17 p. 28 and footnote 29; Nitei Gavriel 56:8

[2] Likkutei Sichos ibid that it requires further research as to whether this is the custom in all places

[3] The reason: Several reasons are recorded behind this custom, including: 1) it is a [mystical] danger for her fetus due to the evil eye; 2) she won’t pay proper attention to the infant she is holding. [Bris Avos 5:26; Toras Alef 3:34] Alternatively, the reason is because we only desire that two people escort the child and not three, and a woman who is pregnant is considered as if she is two people. [Likkutei Sichos ibid]

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

[5] Likkutei Sichos 22:56 [is a custom of Israel], printed in Shulchan Menachem 5:137 footnote 30; 6:206; Minchas Elazar Michtav Oz

[6] Likkutei Sichos ibid footnote 30