Bechorot 47 - How Does One Recognize a Face?
Once a child shows its forehead from its mother's womb, it is considered born; at this moment the lives of the baby and of its mother become equal. However - said Resh Lakish - for the purposes of firstborn inheritance, it is not considered born until his face appears. If he retracts his head and another child appears, the other child will be the firstborn. That is because the word "recognize" is used talking about the firstborn , and for that the complete face must be seen. Rabbi Yochanan says that even here the forehead is enough. Resh Lakish poses multiple questions to Rabbi Yochanan, which the latter answers by emending the rulings. Finally, he answers that perhaps recognizing a firstborn by the forehead is possible while in other situations a full face is needed.
If a woman did not wait for the prescribed three months before remarrying, and her firstborn child does not know the true father, he is the firstborn for the Kohen but not for inheritance. But then he should not get any inheritance, since he can't prove paternity! He should sue the other children jointly with the next son, claiming that between them one is a firstborn.
Art: Franz Cusaude - The Gosnall Twins