Cites a paper stating that tests from 6 weeks’ gestation are 100% accurate.

Development and comprehensive evaluation of a noninvasive prenatal paternity testing method through a scaled trial（Liao Chang et al)

Key points
The percentage of foetal DNA increased slowly in the second trimester and rapidly from the last trimester to birth.
A non-invasive prenatal paternity and paternity testing (NIPPT) method based on a targeted sequencing strategy was developed.
For the first time, the efficiency, reliability and sensitivity of NIPPT were systematically evaluated through a large-scale trial.
The results suggest that NIPPT can accurately determine parent-child relationships from the sixth week of conception to birth.

It says the above. What caught my attention in this paper was that

Only SNPs are used, but 5457 locations must be measured.
In a large study of 349 cases, NIPPT can determine parent-child relationships with 100% accuracy from the 6th week of pregnancy to birth.

This was the case.

Citation.

https://www.sciencedirect.com/science/article/abs/pii/S1872497319300109

Supervisor of the article