Sos tasis ve'sagel ha'akara bi'kibutz ba'neha le'socha be'simcha. Baruch Atah Hashem me'sameach tzion be'vaneha.
"The barren one should rejoice intensely and exalt at the ingathering of her children amidst her gladness. Blessed are You Hashem, Who gladdens Tzion through her children." (Kesubos 8a)
The "barren one" mentioned here, according to Rashi, is referring to Yerushalayim. The Meiri explains that the future joy of Yerushalyim is comparable to the joy of a Chosson and Kalla. We mentions the hardships of Yerushalayim at this happy moment of Sheva Brachos because, as Rashi explains, it says in Tehillim 137:5-6: "Im eshkacheich Yerushalyayim tishkach yemini. Tidbak le'shoni le'chiki, im lo ez'kireichi..." "If I forget Yerushalyim let my right hand forget its skill. Let my tongue stick to my palate..." The remembrance of Yerushalyim is brought up at this point in the order of the Brachos because the next Bracha is the first one that refers to the Chosson and Kalla as a couple (Meiri).
(Chosson and Kalla picture courtesy of mitchellkorman.com)
(Yerushalayim picture courtesy of israeltrip.com)