§ 198 VVG obliges private health insurers to accept newborns and adopted children within the deadline without health questions. The law creates a special right of admission that the parties cannot negotiate away. Insurers can be more generous than the law, not more restrictive.
The deadline is 2 months from birth. What counts is the date on which the insurer receives the application. The previous insurance period of 3 months must have passed at the time of birth, not only at the time of the application. A parent who joined private health insurance 2 months before the birth does not meet the requirement.
Which parent meets the previous insurance period is a free choice. If both parents have private health insurance, they can decide through which parent and which insurer the child is accepted. This affects the child's plan, because in practice the child takes on the plan level of that parent.
Registering in time protects you fully, meaning without health questions, without waiting periods and without a risk surcharge. This also applies to premature babies, children with documented findings from the pregnancy and chronically ill children. The statutory protection applies without conditions, as long as the deadline and the requirement for the parents are met.