1. Steve was captured by HYDRA and is also pregnant at the time. I'm pretty sure Clint and Sam? come to save him with the help of a recaptured Bucky. The baby is Bucky's and I'm pretty sure the baby flinches or Steve sees it that way on the ultrasound machine.
2. Steve is pregnant with Bucky's baby yet again although this time when he gets pregnant he reverts back to how he was before the serum.
3. any and all fics where Steve both tops and bottoms. I think there might have being an A/B/O fic where Bucky is the alpha.
4. any and all fics where Steve is an omega before the serum and an alpha afterwards
4.1. I think there was one where Bucky starts off as a beta and thanks to HYDRA's experiments he becomes an alpha as well. It was set in the 1940's to start with, then 70 or so years later in the future. I can't remember much else except for Tony maybe being an arse.
Please if anyone knows what these fics is called I thank you in advance.