The endometrium is the innermost layer of the uterus that contains the lining which is responsible for accepting and nourishing a fetus during the pregnancy process. The lining of this cavity changes with every monthly cycle in response to the primary female hormones produced by the ovaries: estrogen and progesterone.
If one parent is more stable than the other, judges can decide to award primary custody to that parent. As a result, parental fitness is often a mechanism used in custody battles. Each state has its own set of rules regarding what makes a parent unfit.
The court would be  looking for an appropriate reason to order a change in custody such  as the custodial parent being unfit and the change being in the  best interest of the child.    A child is free to decide which parent to live with in  every state once they have reached eighteen years of age.