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.
In most cases, unless the mother is clearly unfit, the father will want to petition for joint or shared custody or he may want to allow the mother to have full custody with him only having visitation rights. If the mother is unfit to have custody of the child, he will want to petition for full legal custody.