Mozart's first son, Raimund Leopold Mozart, wasn't born alone. Instead, you might say he was one of a set of triplets.
In 1829, while traveling throughout Europe, the musician and music publisher Vincent Novello and his wife, Mary, interviewed Constanze Mozart Nissen about Mozart's personality, life and work. Vincent Novello reported in his and his wife's joint travel diaries that Constanze told them that, on June 17, 1783, while she was in labor with their first child, Mozart was composing the string quartet in D minor, K. 421, in the next room. Some of the passages in the quartet, Constanze said, are musical representations of her labor cries.
There is no evidence that Constanze's story is true. It's also anyone's guess which passages in the D minor string quartet supposedly represent Constanze's screams.
Regardless, Mozart was composing both the D minor quartet and his String Quartet in E minor, K. 428 at the time of Raimund Leopold’s birth.
By 1785 these two quartets, plus the String Quartet K. 387, which Mozart had composed earlier, joined three other quartets to form the set of six quartets Mozart dedicated to the father of the string quartet, Franz Joseph Haydn.
By that time also, Raimund Leopold had died at only two months old, and Mozart's second child, Karl Thomas, had been born. The father of a one-year-old and seemingly with babies on the brain, Mozart crafted the dedication letter he sent to Haydn in September 1785 with the quartets in the form of an extended parent-child metaphor.
"A father who had decided to send out his sons into the great world," Mozart wrote, "thought it his duty to entrust them to the protection and guidance of a man who was very celebrated at the time and who, moreover, happened to be his best friend. In like manner I send my six sons to you, most celebrated and very dear friend. […] Please receive them kindly and be to them a father, guide and friend! From this moment I surrender to you all my rights over them. I entreat you, however, to be indulgent to those faults which may have escaped a father's partial eye, and, in spite of them, to continue your generous friendship towards one who so highly appreciates it" (The Letters of Mozart and His Family, trans. Emily Anderson).