Unfortunately neither the DOL nor the courts have drawn any sharp line as to how long a caregiving relationship must last before an employee can be considered in loco parentis for a child. A child visiting for several months is a close case and may depend on the duration of the visit, the relationship of the child to the employee, the availability of the parents, the age of the child, etc. In the case of an exchange student visiting for an entire academic year, who has limited contact with his or her own parents and for practical purposes becomes the responsibility of the host parents for an extended period of time, I think there is a decent chance that at least the DOL would say that the host parents do stand in loco parentis to the child.

Once the child is born, if the mother will be providing day to day care for the child akin to what a parent might provide, or will provide significant financial support, then she may be acting in loco parentis. If so, the grandchild would arguably be the employee's "son or daughter" for purposes of the FMLA, and she may be entitled to FMLA leave for the birth, to care for the newborn child, and to care for the child due to a serious health condition.