To understand what happens to your body during the pregnancy, you need to know your body.
The uterus, the fallopian tubes and the ovaries participate in the creation of the fetus. The pelvis protects these important organs from trauma. 
As the fetus develops the uterus grows in size. This process can cause different phenomena in your body: 
•    An increase in heart production and in heart rate
•    Frequent urination
•    Nausea, vomiting and heartburns
•    Constipation (sometimes accompanied by the development of hemorrhoids) 
•    Edemas, the appearance of veins on the lower extremities 
•    Hyper-pigmentation - the darkening of the skin in certain areas
•    Back pains
All these changes are temporary, they don’t occur in all women and they usually pass after the birth.