The Marvel Cinematic Universe is the most fascinating and intricately connected of all cinematic universes. So far, the franchise consists of over 20 different movies with multiple characters, all with their own personalities and backstories, who most recently came together in the climactic three-hour Avengers: Endgame.
In the process of building its sprawling world, the MCU earned a large, devoted fanbase, and many hardcore fans have a way of coming up with theories that somehow make the franchise even richer than it already is. One of these fan theories claims that Peggy Carter is actually Tony Stark's mother. The theory totally makes sense in some ways, but in others... not so much. Let's dissect this theory, why it works, and why it doesn't.
The theory was proposed by Reddit user u/savepublicdomain, and their case for this theory begins with one of the earlier Marvel films, Captain America: The First Avenger. We know from this film that Peggy knows Tony's father, Howard Stark. He's a coworker of sorts. Howard even flirts with Peggy periodically throughout the movie. And though Peggy was Steve Rogers' love interest, we know that, ultimately, he crashes into the ice and is assumed to be dead. So, it's possible that, maybe after the heartache had passed, Peggy and Howard got together.
A Peggy-Howard relationship also makes sense when we look at the personalities of each of the characters. Howard is a brilliant, renowned inventor; Peggy is a whip-smart, career-driven, confident woman. Their personalities would balance each other out. Peggy would keep Howard in check, and Howard would probably be attracted to Peggy's confidence and drive. There's also the matter of age. Howard was born in 1917 and Peggy in 1921. They're contemporaries, making it even more likely that they could be a couple.
However interesting this theory is, there are a few factors that make it extremely unlikely. Since Tony Stark was born in 1970, Peggy would have been near 50 years old when she'd given birth to him. Of course, it's still a possibility that she could have had a kid then. But, factor in how much time had passed since The First Avenger when she'd initially met Howard, and it becomes even more questionable.
That movie takes place in the 1940s, so if Peggy had Tony in 1970, that means she and Howard would have had to meet up nearly 30 years later. While we know from Endgame, they worked in the same S.H.I.E.L.D. facility then, we also know that Peggy wasn't pregnant at the time, whereas Howard's wife was, making it clear that Maria Stark was Tony's mother, not Peggy.
On top of this, while we know Peggy had two kids, Tony Stark wasn't one of them, and their father wasn't Howard. In fact, according to Christopher Markus and Steven McFeely, who created the series Agent Carter and have been writers for several MCU films, it was always their intent that it was Steve who fathered Peggy's kids, although not everyone involved in the MCU agrees with their perspective.
Either way, the idea that Peggy Carter could be Tony Stark's mother is fascinating and would add yet another mind-blowing twist to the MCU. However, it's not canon, and it's highly unlikely.