the following has to do with naturalization process - not citzens at birth, however, the concern over the term ‘continuous physical presence’ doesn’t necessarily mean they must not leave the US during this time:
Physical Presence
Applicants are required to show that they were:
Physically present in the U.S. for thirty months within the five year period before applying, or (see legal basis)
Physically present in the U.S. for eighteen months within the three year period before applying in the case of qualified spouses of U.S. citizens (see legal basis)
continuous presence requirement:
Absences of more than six months but less than one year may disrupt an applicant’s continuous residence unless the applicant can prove otherwise, (see legal basis, see policy manual)