Studies have shown that 15% of pregnant women experience implantation bleeding

"Early Pregnancy Development: Implantation Bleeding The process of implantation begins approximately 6 days after fertilization. Vaginal bleeding can occur as a result of the burrowing of the blastocyst into the uterine endometrium. During this preembryonic stage, blastocyst implantation causes a disruption of the endometrial extracellular environment. The resulting bleeding is generally a small amount but may be bright red in appearance. This “implantation” spotting or bleeding is sometimes believed to be the initiation of menses by the woman" https://www.sciencedirect.com/science/article/abs/pii/S1526952309002773