Boardwalk Empire

Take it with a grain of salt, but here is what the Wiki for Boardwalk Empire says about Mabel. I guess you can presume that the miscarriage took place first and could have been setting the tone for how she dealt with things later on:

http://boardwalkempire.wikia.com/wiki/Mabel_Thompson

Mabel was born on May 6, 1877. She married Nucky Thompson some time before 1896. Their first and only son Enoch Thompson Jr. was born just before Christmas in 1897. He was sickly and underweight, most likely premature. She was determined to nurse him to health but she had little support from her husband, who was terrified of the frail baby, afraid to hold him or even look at him. Nucky avoided them completely and put everything into his new job as county treasurer.
A week after the birth, Nucky returned home late and finally found the courage to look at his son, at which point he immediately realized the baby was dead. According to him, Mabel had “completely broken with reality", caring for the infant's corpse for days as if he were alive.

Enoch Jr. and Mabel's tombstone, as seen in A Return to Normalcy.
Mabel was diagnosed with melancholia shortly before or after Enoch Jr.'s burial. Doctors assured Nucky that she would recover with time, but Nucky knew better. He later admitted that he knew Mabel needed him, but he was "so very, very busy.” Mabel, left alone once again while her husband worked, slashed her wrists with his shaving razor a few weeks after the baby's burial, on January 19, 1898. ("A Return to Normalcy").
Nucky is ashamed of the whole incident and whenever he is asked about his wife's death he claims that she died of consumption, a fate that actually belonged to his younger sister, Susan Thompson ("Georgia Peaches"). The only person he has ever confided the truth to is Margaret Schroeder, who he told about in early November 1920 ("A Return to Normalcy").
Ethan Thompson also brings up Mabel from time to time when he confronts his estranged son, jokingly asking if she is whoever he is dating at the time. Needless to say, Nucky is not amused. ("Home")
 
I don't know if they ever mentioned how Nucky's mother died. I assume natural causes but who knows. I suspected the boy was Tommy also. And agree he did remind me of Jimmy. I thought the actors who played young Nucky, Eli and Gillian also did an amazing job. They looked and sounded so much like them! Regarding Tommy it also makes me sad as Richard went through so that he could be away from Gillian and have a good life. So I too am curious what happened. Very sad to think now he will spend his life in jail probably in order to get justice.


Shivers about the doctor!!! Yikes!


I swear I recall in season one or season 2 Nucky telling Margaret about Mabel having a stillborn that she cared for after for days or a week, something like that. Essentially she had lost her mind and committed suicide. This miscarriage had to have been prior to this. And thus the miscarriage along with stillborn birth and post partum caused her to go off the deep end with depression