Brown Spotting for days before Period

For the last 4 months I have been getting brown spotting for days before my period starts. It starts of light watery brown and gets darker until the day my period starts. It is usually about 4-5 days before my period, so it's almost like a warning that my period is coming, which would be great if it was one day before, but not 5! 
We are TTC, so I am little worried what it is. Does anyone else get it? 
I have a retroverted uterus if that makes a difference. 
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