Bechorot 48 - Firstborn Both for Inheritance and the Kohen
One who is a firstborn both for his father and his mother is an obvious case that need not be mentioned. Rather, the first case of this ruling is when a woman has a miscarriage, but an embryo is so undeveloped that it looks full of water or multicolored matter, like a fish or a crawling animal, or if she aborts it on the fortieth day after conception - all these are not considered births, and the next child is a firstborn both for inheritance and for the Kohen.
A firstborns son born by Cesarean section and the naturally born that follows him - neither is a firstborn for either inheritance - because the Torah said " And they bore him sons ," and Caesarean is not a classical birth, or the Kohen - because the child needs be both the first and to open the womb. Rabbi Shimon considers the first one firstborn for inheritance, because in his opinion the Caesarean section is legally included in giving birth. He also regards the second one a firstborn for giving the Kohen five silver coins, because he considers either of the two conditions - first birth or first to open a womb - sufficient.
Art: Nicholas Gysis - New Arrival