Bekhorot 19: three things to remember

Rabbi Akiva rules that neither a caesarean offspring nor its subsequent sibling is sanctified, because a firstborn must satisfy both conditions.
Rabbi Akiva holds that uterine discharge or afterbirth exempts subsequent births from bechor status, even before an animal delivers a live offspring.
The Rabbis rely even on statistical majorities, permitting minor orphans to enter levirate marriage without fearing they are sterile.