Sheldon (Jim Parsons) may have caused Penny's (Kaley Cuoco) pregnancy in The Big Bang Theory. For years, CBS stagnated the Hofstadters' arc in the sitcom to focus on other stories. In an effort to rectify this in The Big Bang Theory's final season, it introduced a marital issue for the pair after Penny revealed that she didn't want to have kids, whereas Leonard (Johnny Galecki) was adamant about growing a family with her from the day they met.
Throughout their years of dating, this never became an issue for the couple. Despite the poorly established conflict, The Big Bang Theory moved forward with the arc, stretching out the plotline and introducing other factors that made it more complicated. For some reason, however, Leonard and Penny didn't have any extensive discussions about the matter. Instead, Leonard simply compromised and didn't push back on his wife's decision, although he clearly wasn't happy about it. It was The Big Bang Theory's opportunity to tackle a grounded issue within its comedic setting. Unfortunately, they didn't take that chance. Instead of properly exploring the nuances of the situation, they came up with a last-minute resolution with the reveal that Penny got accidentally pregnant.
The revelation that Penny and Leonard were expecting came during The Big Bang Theory finale. Apparently, they conceived their first kid after she spent the night drinking with Sheldon on the heels of his Nobel Peace Prize in Physics win with Amy (Mayim Bialik). Beyond that, it's uncertain what Penny and Leonard's initial reaction was upon learning about the big news, but it appeared as if both were thrilled despite her initially being against it. This poses some lingering questions that remain unanswered, but one particular theory could explain what actually led to the pregnancy in the first place.
As one of the main romances in The Big Bang Theory, it was obvious from the get-go that Penny and Leonard were going to end up with each other. So despite various complications, there was always the idea that they would make it through the sitcom together. Despite this, they never really got the chance to discuss specifics of the life they wanted to have, including whether or not they wanted to have kids. So, it became a big disagreement when they realized years into their marriage that they had conflicting thoughts about it. Penny was scared that getting pregnant could negatively impact her life. After struggling in her career for the longest time - with Penny even becoming a bartender in The Big Bang Theory season 4 - she was finally thriving in the pharmaceutical industry. Seeing how Bernadette (Melissa Rauch) struggled to re-establish herself in their company after getting pregnant, Penny's fears were not without merit.
Leonard wasn't the only one who didn't like that Penny didn't want kids in The Big Bang Theory. Upon learning about this, Amy dramatically stormed Apartment 4A and confronted her best friend about it, saying that the idea was that the Hofstadters' kids would guarantee that her and Sheldon's babies would have automatic friends, as both Amy and Sheldon struggling with gaining and maintaining friendships throughout their lives. While this isn't assured considering that Penny and Leonard's The Big Bang Theory babies could have different preferences than theirs, the pair could guide them to get used to Amy and Sheldon's kids. In addition to that, their proximity with each other would mean that their kids would have to start spending time with each other starting from a very young age, making it so much easier to foster a friendship.
Knowing Amy's stance on Penny's pregnancy decision in The Big Bang Theory, it isn't outside the realm of possibility that Sheldon intentionally invited Penny out in the hopes of her getting pregnant - though this would be one of the darkest moments of manipulation that the character has had if this were to be the case. Indeed, Sheldon was genuinely struggling with the fact that he just achieved his biggest dream which was to win a Nobel Prize, so the case may have been that he simply wanted his friend's support. Considering the fact that all of this happened in the last two episodes of The Big Bang Theory, it didn't have the time to deal with the aftermath of Penny and Leonard's pregnancy news. Aside from tackling Sheldon and Amy's Nobel Prize in Physics win, the show also had to devote screentime to wrapping up Howard (Simon Helberg) and Bernadette's narrative. This is perhaps for the best, as having Sheldon be in any way involved in the accidental pregnancy storyline would have finished the show on a sour note.
Although The Big Bang Theory finale showed Penny and Leonard being both happy about the pregnancy, it could actually be a source of friction for them moving forward. As heartwarming as it was to see the couple happy with the news that they were expecting their first child, Penny had been previously adamant about wanting to be child-free, and from the looks of it, they never got the chance to properly resolve this. Despite her reaction in The Big Bang Theory finale, there's still a chance that she struggles with raising a child, as her fears surrounding this topic were never fully abated. Leonard, on the other hand, could be oblivious to this complex scenario, as he did always want children. This could result in resentment from Penny and conflict between the pair. Worse, it could even be enough to lead them to separate, as the issues they had about having children isn't likely to have been solved simply by the accidental pregnancy itself. This would be a bigger problem for Sheldon and Amy as it would put them and the rest of the Pasadena gang in a difficult situation dealing with the split of the Hofstadters - hopefully suggesting Sheldon didn't manipulate Penny in the first place, as not only would it have been a callous move, it also would have had a higher chance of sabotaging the group's bonds.