<P> In season 12, Abby rekindles her relationship with Luka in the episode "The Human Shield" when her attempt to vent her frustration at him turns into a kiss and subsequently sleeping together . Initially, they decide to remain friends, but soon become lovers, noting that they always seem to find their way back to each other . Three weeks into their relationship, it is revealed that Abby is pregnant . Despite clearly wanting another child, his own having perished in Croatia, Luka states that he will support whatever choice she makes, as long as they can still remain together . After a week or so of uncertainty, Abby decides to keep the baby . Luka proposes to Abby while she is pregnant, but Abby is not comfortable with the idea because of her past problems, and later claims she' doesn't need a white dress' to prove she loves him . Their baby, a son named Joseph (after Luka's father), or Joe for short (after Joe Frazier, the boxer whom Abby's long - disappeared father was a fan of) is born prematurely by emergency C - section at the beginning of season 13 . Abby has to have an emergency hysterectomy . Joe spends several weeks in the hospital's NICU, requiring surgery during his stay . Abby and Luka's relationship becomes strained during this period, but they appear much happier in later episodes, caring for their now healthy baby boy . </P> <P> Throughout season 13, Abby is shown growing into her role as Joe's mother, first by taking time off to be with her seriously ill baby, then by participating in activities like Mommy and Baby classes and walks in the park, and then later by gradually reintroducing herself into the ER world . Mid-season, tension develops between Abby and Luka, partly as the result of Luka's apparently paranoid behavior regarding Curtis Ames (Forest Whitaker), a disgruntled former patient who has sued Luka due to a stroke he suffered that cost him the use of his arm . Not realizing that Ames approached her and Joe in the park posing as a friendly stranger, and has been stalking both her and Luka for some time after he lost the case, Abby brushes off Luka's concerns . However, Ames breaks into Abby and Luka's apartment with a gun, and demands to see Luka . Abby tries to bargain with him, telling him that she is a recovering alcoholic, and recounting an occasion where she felt she had hit' rock bottom' . Luka is taken hostage, threatened and has his hand crushed by Ames, but survives when Ames commits suicide . In the next episode, an emotional Abby asks Luka to propose to her again . She says if they got through the shooting incident and the Curtis Ames nightmare, they can get through anything - he proposes and the couple get engaged . Later Luka realises that Abby is hesitant about planning the wedding, so he decides to prepare a surprise wedding for her . At first, she has her doubts but eventually she agrees and asks Neela Rasgotra to be her maid of honor . </P> <P> At the end of the thirteenth season, Abby and Luka plan to honeymoon in Hawaii . However, the night before they are due to depart, Luka receives a phone call from Croatia informing him that his father is ill, so the honeymoon is put on hold until Luka can return from his urgent trip abroad . After Luka's departure, Abby has a tough time with the stern new ER Chief Kevin Moretti, who plans to mold the department into a more efficient and professional work environment, but Moretti quickly tells Abby he is very impressed with her work . Despite this, she seems to dislike him more than many of the other residents . In season 14, Abby is struggling to deal with Luka's absence, raise Joe and work at the ER; in "Gravity", Joe is injured after falling off a jungle gym at the playground and Abby ends up falling back into drinking at the episode's end . She approaches Dr. Janet Coburn (the O.B. Chief) who appears to have been her AA sponsor in the past . Abby's drinking problem continues to escalate, and she realizes how bad it is in "Blackout" when she shows up drunk to a party in honor of Pratt and Morris passing their boards . Abby finds herself waking up at 3am in Moretti's apartment realizing, after sobering up at the airport when trying to find a flight to Croatia to see Luka, that she slept with him . </P> <P> Soon after Luka arrives back in Chicago, he gets news that his father has died . Abby tells Luka to take Joe to Croatia for the funeral while she seeks help for her alcoholism - though she does not initially reveal the infidelity that her blackout caused . Upon her return to the ER, Abby faces the reality of her situation; she must tell her co-workers the truth about her disease . The only negative reaction at work is from Sam (Linda Cardellini), who displays disgust at Abby's weakness, but the two women mend fences when Sam tells Abby she was being harsh because of her own long history of being around alcoholics who refused to clean up their lives . Abby goes to Croatia and confesses her affair to Luka . Luka is angry that she cheated on him and put their son's life in danger . When they return to the States, he decides to move out of the apartment, though he does not seek separation or divorce papers . He decides to work at a long - term care facility and soon has a change of heart about Abby's mistake, especially after a tense conversation with Dr. Moretti . Moretti also attempts to apologize to Abby, but she coldly rebuffs him . Abby and Luka embrace at the end of the 14th season . Abby tells Luka how much she loves him and they kiss, with Luka forgiving her . At the conclusion of the 14th season, it would appear that Abby and Luka are willing to continue to work on their marriage . Abby's happiness at this is tempered when the ambulance she walks past explodes, jarring her and mortally wounding her friend and colleague Dr. Gregory Pratt . </P>

Who did abby on er have an affair with