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.
These arrangements can allow for both physical custody and legal custody to be shared in any way the parent(s) and grandparent(s) see fit. If a grandparent believes that the parents are unfit, some states will allow a grandparent to seek full custody.