Lyric discussion by Banjospeed 

I know it sounds weird, but the first time I heard that song i thought its meaning is about motherhood.

"I felt you in my legs before i ever met you" matches to the relationship between a mother and her unborn child.

"and now we're saying bye, bye, bye" could describe the situation when her child is grown up (or dead? a stillborn?)

with the line "I was nineteen" she remembers the pregnancy when she was nineteen.

P.S.: I'm a man and not pregnant. And I know the actual meaning won't be about motherhood (since tegan and sara are both homosexual).

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