A revised Japanese law to address what are deemed as outdated rules on paternity took effect Monday, taking a more flexible approach to designating the legal father of a child born after a divorce and removing a 100-day remarriage ban for women.

The revamped Civil Code maintains that if a child is born within 300 days of its mother's divorce, the previous husband will be the child's presumed father. But under the new provision, if a child is born at the point when the mother is remarried, the new husband will assume the legal paternity as an exception to the 300-day rule.

The first such change in the 126-year-old Civil Code provision on the presumption of paternity came as many women have opted not to submit notification of the birth of a child with a new partner, hoping to avoid having their former husband recognized as the legal father of the child.

Some of them have fled domestic violence from their former spouses or were still undergoing divorce proceedings when they conceived a child with their current partner.

But leaving children off their family registry by failing to report their births to local governments puts them at a disadvantage in receiving a wide range of public and private services in Japan, including matters related to marriage, education and employment.

The original Civil Code stated that a child born 200 or more days after a marriage or within 300 days of the date of divorce are presumed to have been conceived within the marriage. The rule was intended to protect the child's welfare by swiftly determining the child's legal father.

The provision on the 100-day waiting time for women to remarry, meanwhile, had been in place to avoid the overlapping of claims for legal paternity by the former and current husband in the event a woman remarries shortly after divorce and gives birth between 201 and 300 days later.

But the ban has been criticized as discriminatory for only applying to women and has become unnecessary with the latest change.

The revised Civil Code, which was enacted in 2022, also expanded the right to file for rejecting paternity for children born within 300 days of a divorce to mothers and children. Previously, the right to challenge paternity was limited to fathers.

The period for the filing has also be revised to within three years of knowledge about a birth from the previous limit of within one year.

Children born prior to the revised Civil Code coming into effect will also be retroactively covered during the first year of its implementation.

But despite the legal changes, critics have said their impact on addressing the issue of children with no family registry may be limited, as they leave out mothers who cannot get a divorce in the first place, or those who choose common-law marriage rather than a legal marriage with their new partners.

For mothers wishing to ensure that their child born within 300 days of a divorce belongs to their current partner, the Justice Ministry issued a notice in 2007 approving such treatment if a doctor's certificate proves that the mother became pregnant after her divorce.

But that relief step is also said to have limits as it does not include mothers who got pregnant before a divorce.

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