Past Life Returner - Chapter 201

To treat her sick brother, she was forced to marry him in place of her cousin, who was disfigured and paralyzed. At first, their relationship was purely transactional, with neither caring much for the other. But after going through a lot together, they fell in love with each other. He doted on her deeply.She was considered an ordinary person by the world, but it was later discovered that she had multiple identities - a mysterious underground boxing champion, a highly-skilled doctor, and a mysterious and renowned composer... He also realized that their fates had been intertwined long before they met.

Childhood sweethearts.
The most popular person in school is my childhood sweetheart.

- Description from Novelupdates

"An accident. She's pregnant. How unlucky. She got pregnant in one shot!