I have experience of two sides of this.
In 3 yrs of trying for no.1 I had 3 bfp's (2 x mc- for both mc I had implantation spotting) it was brownish and in one are lasted a week) never heavy enough to really need a tampon iykwim. Before ttc I had had pre period spotting since coming off the pill and had asked dr's all said no problem but before I had any pregnancies I manged to get rid of the spotting. I think although it could be random it was taking b complex. It's meant to help with progesterone???
In fact to get my successful bfp I upped the dose to the h&b b50 as I had discovered a lutuel phase defect. Within 6 weeks of taking it I had a bfp - with no implantation bleeding which resulted in ds. I don't think all pregnancies with implantation bleeding end badly as the dr on both occasions said the bleeding was quite normal. From what i was told The theory of implantation bleeding seems to be twofold 1, as egg burns into lining or 2, a dip in the change over of hormones so af begins a bit with old (brown blood) but the rises again. I also had a friend whose early pregnancy bleeding was caused by a fibroid. So there are a number of causes.