The Torah establishes in the Book of Devarim (21:17) that a father must "recognize" ("Yakir") the distinction of his firstborn son by awarding him a double portion in the inheritance. Thus, for example, if a man had three sons, after his death his estate is divided into four portions, two of which are awarded to the firstborn; each of the other two sons then receives one of the remaining two portions. Interestingly enough, the Hebrew word "Bechor" ("firstborn") itself alludes to this Halacha. This word consists of the three letters "Bet," "Chaf" and "Resh," which have the respective numerical values of 2, 20 and 200 all of which signify the notion of "double."
This Halacha applies only if the deceased's first child is male and born naturally (not cesarean.) If his oldest child is a daughter, then the oldest son does not have the status of "Bechor" and does not receive a double portion of the inheritance. Furthermore, this Halach only applies to the first born of the father, not the mother.
The right of the firstborn to a double portion does not apply if the father died, Heaven forbid, before the oldest son's birth. Meaning, if a woman conceived with twins and her husband passed away during pregnancy, the estate is divided evenly between the two twins. As mentioned earlier, the Torah presents the law of the firstborn's double portion with the word "Yakir," which means to "recognize" or "know." This term alludes to the fact that a firstborn receives a double portion only if his father "knew" him," to the exclusion of a tragic case of a firstborn whose father passed away before his birth.
If one's oldest child is a son but he was born after a miscarriage, he is nevertheless considered a "Bechor" with respect to this Halacha. Even though a firstborn son after a miscarriage does not require a Pidyon Ha'ben (the symbolic "redemption" of a firstborn son from a Kohen), he is deemed a Bechor as far as inheritance is concerned. A firstborn son thus receives a double portion even if he was born after a miscarriage or several miscarriages, and even if he was born after a stillborn child (or several stillborn children), Heaven forbid.
These Halachot apply only in cases where no will was written, and thus the deceased's estate is distributed automatically in accordance with the system prescribed in the Torah. We do not discuss here the issue of if and how one can circumvent these laws by drawing up a will during his lifetime.
Summary: When a fathers naturally (not caesarean) firstborn child is a son, he receives twice as large a share in his father's estate as his brothers. This applies even if the firstborn was born after a stillborn or miscarriage. It does not apply in a case of a man who dies while his wife was pregnant with twins; since he died before the birth of his firstborn son, the son does not receive a double portion.
See the book- "Pure Money" by Dayan Cohen, pages 222-223. See Hoshen Mishpat, siman 276.
