Regardless of the cause of the threatened abortion, miscarriage always develops progesterone deficiency - a lack of the hormone progesterone that preserves the pregnancy, or a decrease in the sensitivity of the uterus to its action.
The blood flow in the uterus is disturbed, especially in the area of attachment of the fetal egg, with the development of thrombosis of the vessels of the placenta.
In the case of an infectious cause of miscarriage, the damaging effect of inflammatory mediators is added to all of the above.
Under such conditions, the fetal egg cannot attach to the uterus, and its development is disturbed. It either dies or is expelled from the uterus.