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Spotting before regular periods is normal at times.It can be due to the hormonal changes occuring before periods.If you have been having regular intercourse during ovulation/fertile days and still not conceiving then it requires a careful evaluation.
Consider visiting a Gynaecologist for examination and to rule out any underlying disorder as PID,endometritis,cysts etc which can cause problems in conceiving.You might require USG scan and also get your partner's Sperm Analysis done.
Take Care!