Short answer. Whoever is making the claim carries the burden of proving it. If you allege the child is legitimate, you must prove that; if someone else alleges the child is illegitimate, the burden of proving that falls on them instead. The law does not presume either status once 300 days have passed.
What the law says
The legitimacy or illegitimacy of a child born after three hundred days following the termination of the marriage shall be proved by whoever alleges such legitimacy or illegitimacy.
Family Code, Article 169 — Children Born Beyond 300 Days After Termination. Read the full provision →
No automatic presumption once the 300 days have passed
Article 169 applies specifically to a child born after three hundred days following the termination of the marriage. Beyond that point, the law does not automatically treat the child as either legitimate or illegitimate. Instead of a default status, the article puts the question squarely on proof — whichever status is being claimed has to actually be established, rather than assumed from the timing alone.
The burden follows whoever makes the claim
The rule is symmetrical: whoever alleges legitimacy or illegitimacy is the one who has to prove it. It does not matter which side of the claim you are on — if you are the party asserting the child's legitimacy, that burden is yours; if another party is asserting illegitimacy, that burden belongs to them instead. Neither side automatically gets the benefit of a starting presumption in their favor here.
Why this differs from the ordinary marriage-based presumption
For a child born during the marriage or within a shorter window after it ends, the law generally leans toward presuming legitimacy without demanding proof upfront. Article 169 marks out a different zone — beyond 300 days, that presumption drops away, and the passage of time itself is treated as enough of a gap that the question has to be proven rather than assumed either way. The 300-day figure is not arbitrary. It echoes the outer edge of the gestation period the Family Code uses elsewhere: Article 166 lets legitimacy be impugned by showing sexual intercourse was physically impossible in the first 120 days of the 300 days immediately preceding the birth. Article 169 picks up where that same 300-day span ends, treating a birth beyond it as too far removed from the marriage for either legitimacy or illegitimacy to be assumed automatically.
What this article does not tell you
Article 169 fixes who carries the burden of proof; it does not itself specify what kind of evidence is sufficient to prove legitimacy or illegitimacy in a given case. What will actually satisfy that burden depends on the specific facts involved, and this article should be read as the allocation-of-proof rule rather than a checklist of acceptable evidence or a predictor of how any particular case will turn out.