I knew this was a fairly common story, so looked up the term for it. Found there is a TV Tropes page for Tzadikim Nistarim (36 righteous men). On there, found this:
Sam Bourne's book The Righteous Men is centred around a plot to kill the Tzadikim Nistarim and thus end the world.
No modern examples of the 36 exhibited mystic powers, but they each help out their fellow human beings, from one performing individual acts of kindness to neighbours, to another who blackmarkets a medication for HIV that will help millions around the world.
Sometimes some subconsciously try to obscure their altruistic nature so much that they are often criminals due to fate or in their attempts to help others. One member of the 36 did become aware of his nature, and did not die instantly however. Additionally one member of the 36 at a time is determined to be the Messiah if the End of Days occurs while he is alive, and a particularly spiritual and pure Righteous Man will attract others to him, and even be able to recognise his fellows.
Members of the 36 mentioned include a pimp who sold all his belongings to save a woman from entering prostitution, a medieval pimp who could pray for rain, a teenage hacker who invents a virus that will eventually destroy all child pornography on the internet, and a simple nurse in Africa who saves war refugees
Also, an Amazon review had this:
In addition to a really complex mystery was the fascinating world of the Hasidic community, including the customs, practices and history of the Jewish faith.
Goodreads summary mentions the main character's ex, TC.
Desperate to find the link between the killings and to save his wife, he enlists his college sweetheart, TC, an eccentric artist and Kabbalah expert.
One review on Goodreads mentioned a pregnant woman.
The sense of urgency was a bit difficult -- for some people in the story, it was the end of the world; for the sceptical reader, and for the main character, it was "just" one woman and her unborn child.