In this type of fanfiction, Natasha could be injured during a fight and later find out she's pregnant. Steve would be extremely concerned for her and the baby's health. They would start to plan for a future outside of just being Avengers. Natasha might have to cut back on her dangerous missions. The pregnancy could also lead to some revelations about their own pasts and how they want to raise their child differently. When the birth arrives, it would be a moment of pure happiness, and they would name their child after a fallen comrade as a way to honor them.
Well, perhaps Natasha and Steve fall in love during a long and arduous mission. Their relationship deepens and soon Natasha realizes she's pregnant. Steve is filled with a mix of excitement and worry. They might have to keep the pregnancy a secret from some of their enemies at first. As the pregnancy progresses, they could face some internal conflicts within the Avengers team, with some members being concerned about how a baby would affect their dynamic. But in the end, the birth would bring the team closer together as they all celebrate the new life.