1 Chronicles 5

Now the sons of Reuben the firstborn of Israel—he was indeed the firstborn, but because he defiled his father’s bed, his birthright was given to the sons of Joseph, the son of Israel, so that the genealogy is not listed according to the birthright;  yet Judah prevailed over his brothers, and from him came a ruler, although the birthright was Joseph’s—  vv.1-2

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Next starts the genealogy of the other tribes of Israel starting with Reuben the oldest son, the first born.  The oldest son who was to receive the birthright which denotes special privileges and advantages than his brothers as well as a double portion of the father's inheritance.  Yet Reuben forfeited his birthright because he defiled his father's bed by sleeping with his fathers concubine.  The birthright was then given to Joseph yet even then that wasn't the tribe Jesus came through.  The messiah, this ruler mentioned here, came through the tribe of Judah, this was not according to having a birthright or not but due to more than what deck of card life dealt you.  A higher privilege was given than having the birthright and that was given by God for His own reasons.  Gill's Exposition on it says, 

"the tribe of Judah prevailed above the rest in number, in valour, and courage, and in dignity; wherefore the genealogy is not reckoned according to birthright, but dignity and dominion."

Middlechild syndrome didn't stop Judah from having a dignity and power of his own that God ultimately blessed by using his line from whom came the Messiah, Jesus Christ.  
Are you a middle child?  If so how does this give you hope?  If you're not a middle child, how have you taken advantage (for good or bad) of being the oldest or youngest or only child in your family?