I was on Pinterest looking for funny Sherlock pins and I found something really cool. Basically in Signs Of Three, there's a bunch of references to the magpie. During Sherlock's best man speech, the walls behind him have three magpies and according to the Magpie poem/rhyme, three for a girl and also three for a wedding. Later, Sherl informs John and Mary that Mary is pregnant and in His Last Vow John tells him that he is expecting a daughter. Also, including Moriaty's seal and the song he listened to during his break-in, there are seven references to the magpie and in the poems , seven for death.