Myths of Human Genetics (2011)
udel.edu
udel.edu
BTW the technology exists (and have been sold for nearly 20 years) to determine paternity, sex and genetic diseases in an infant 2-6 weeks after CONCEPTION from the mother's blood.
The same applies to other cases where parents aren't genetically related to their children (i.e., donor egg and/or sperm).
[1] https://www.theatlantic.com/family/archive/2019/07/adoption-...
Whether or not the parents had a paternity test performed wouldn't have any bearing on it.