Day 90 of pregnancy:[1]

It is forbidden to have intercourse on day 90 of pregnancy, due to danger to the fetus as it can cause miscarriage.[2] Nonetheless, in today’s times, one is not required to be stringent in this matter, and Shomer Pesaim Hashem.[3] [However, according to Kabbalah, this stringency applies even today.[4] Accordingly, some Poskim[5] conclude that it is proper to be stringent even today to avoid intercourse on the 90th day from her last Mikveh night, during pregnancy. However, there is no obligation to specifically calculate the 90th day and only if one happens to know it, is it proper to abstain.]

[1] M”A 240:5; See Nitei Gavriel 103:2; Piskeiy Teshuvos 240:8; Sheyikadesh Atzmo 13:5; Sefer Shemiras Haguf Vihanefesh [Lerner] 127:8

[2] M”A 240:5; Niddah 31a; Menoras Hama’or Ner Gimel Klal Vav 5:1; Zohar Shemos 3; Peri Eitz Chaim 16:11; Oar Tzadikim 27:1; Kaf Hachaim 240:22

The reason: One who has relations on this night is considered a murderer, as doing so can kill the child. [M”A ibid in name of Zohar ibid]

[3] M”A 240:5; Abayey in Niddah ibid

The reason: As it is not possible to know definitively which day is the 90^th day of pregnancy. [Niddah ibid] As not always does she become pregnant right after Mikveh. [Aruch Laneir Niddah ibid; Noda Beyehuda; Mishneh Halachos 11:193]

[4] Peri Eitz Chaim ibid, brought in M”A ibid; Kaf Hachaim 240:22

[5] Shevet Halevi 3:113; Piskeiy Teshuvos 240:8; Nitei Gavriel Niddah 2:103 footnote 3; Sheyikadesh Atzmo ibid