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The point where the entire weight of any object is concentrated is considered as the Center of Gravity.

In the case of a normal human, the center of gravity of the body is at the stomach, along with the vertical alignment of the legs.

But in the case of pregnant women, there is extra weight in the stomach, due to which the center of gravity shifts in a forwarding direction. This would create an unbalanced body and make women fall in the forward direction.

In order to coincide the center of gravity towards the line of vertical alignment, the pregnant women have to tilt her upper body to the backside. This would shift the center of gravity in the proper position and would not let her fall.

Hence, this is the required reason.