It's an accident.

You were his new biographer after Dr. Watson. One day he came to the flat heavily drunk while you were writing, and snatched you to his bedroom forcefully.

Right now, he was caressing your midriff as you were sobbing on his lap, as your father threw you away after knowing you bear someones child inside you -- Sherlock's child.

"Shh, it's all my fault, I'll be responsible." Sherlock whispered as he kept caressing your midriff gently.