I think that Shilpa was already pregnant before Harshad and Shilpa's were married. The reason why I think that is because Motabhai said that if the raaz comes out then someone is going to be baadnaam and also because like when Lela and Pravina were trying to read Baa's diary. Baa told then that in this house there is one bhai who had a najays bacha. And one more this I noticed is that when Shilpa told her part of the raaz it kind of involved something about baadnaami.....
Sorry If someone has already posted something like this.