I haven't experienced it and hadn't heard of it so i looked it up and found this
"Decidual bleeding is one of the most common causes of period-like bleeding during pregnancy. The fact is that sometimes, during pregnancy, bodys hormones can get malfunctioning, causing to lose parts of the lining of uterus. This is especially common in the early stages of pregnancy, before the lining has completely attached to the placenta, which is the most common time to notice decidual bleeding. However, decidual bleeding is generally not thought to be a health threat to you or your baby."