Oldest, but Not Firstborn
In commemoration of Makas Bechoros, where all the firstborn sons were killed overnight, the Torah commands us to sanctify the firstborn son of each household. Originally, they served in a priestly capacity, the religious leaders of the generation. However, after the sin of the golden calf, their position was transferred to the Kohanim and Leviim, who did not participate in the sin. Instead, the Torah commands us to redeem the firstborn for 5 silver coins. The Torah categorizes a firstborn as “peter rechem” or the one who “opens the womb”. Accordingly, not every firstborn has the Halachic status of a Bechor.
The question in Halacha is what stage of development is required to be in itself a peter rechem and thus relieve future pregnancies from being a Bechor. The Gemara discusses important phases in pregnancy: the first forty days which is not yet considered a fetus, and after 3 months, at which point the Gemara assumes that the pregnancy is visible and therefore solidified.
The most ambiguity lies between the period of 40 days and 3 months. What does Halacha assume about the developmental timeline within that period? The answer to this question lies in a technical Machlokes between the Maharik and the Chacham Tzvi about the application of Rov, Chazaka, as well as our ability to discern the exact development required in Halacha to render the stillborn a child. The modern Poskim rule in accordance with both opinions- to redeem a subsequent firstborn male, but without a bracha, out of respect for the possibility that this child may not be a Halachic Bechor.
Finally, R’ Shlomo Zalman ruled that one can rely on ultrasounds prior to the miscarriage to determine the stage of rikum eivarim- the formation of limbs that renders a fetus a “peter rechem”, but once the miscarriage has occurred, we cannot rely on technology or pathology to determine this stage. Rabbi Sprung explains that this is in accordance with the Chacham Tzvi who worries for posthumous changes that make it impossible to properly assess the child from a Halachic viewpoint.
Adapted from medicalhalacha.org