Hilchos Bikkurim 11:14

Let’s say that a non-Jewish woman or a maidservant gives birth and then converts or is freed, after which she gives birth again. In such a case, the second child need not be redeemed because he’s not the first issue of his mother’s womb. The same is true if a son is born after a miscarriage.* In a case where the mother is ritually unclean because of a miscarriage, a son born after is not “the first issue of the womb.” If a miscarriage doesn’t render the mother ritually unclean, a son born after must be redeemed. Examples of the latter are if the miscarriage resembles a fish or a grasshopper, if a woman miscarries within the first forty day after conception, etc.

Hilchos Bikkurim 11:15

If a fetus in the womb is aborted and removed limb by limb, a son born subsequently is not the first issue of the womb. If the head of a fetus that was carried for eight months came out alive and was then pulled back into the womb and died there, or if the head of a stillbirth that was carried for nine months came out and went back in, and then the twin brother of that fetus was born viable, the latter is not considered first issue of the womb. This is because all later births became exempt from pidyon haben when the head of the first fetus came out. When the forehead appears, all those born after need not be redeemed.

*Including abortion and stillbirth.