SMy mother replied: "Because I was not pregnant at that time." I was suddenly dumbfounded: isn't she and thought to myself isn't this woman my real mother?

My momShe added: "At that moment, I heard the sound of an explosive souon, and, then the fire wasspread everywhere. I tried ....

I combined these two sentences because you are still describing what your mother is saying.

I tried.... to escape from that building, but I found I was pregnant and was goingit was difficult as I was about to give birth."

Shortly after, a fireman saved me and took merushed my mother to the hospital tso she could give birth, and that baby was you."me.

This sounds like it is the fireman that is giving birth to you. I changed it for you.

I said to my mom: "This couldn't be impossible, according to the biological rules."

I'm not sure what you are trying to say. Sorry.