What puts someone at risk for bleeding during pregnancy?

Multiple factors. Just being pregnant gives you a 1 in 3 chance for spotting or bleeding during the first 3 or 4 months of pregnancy. This is usually due to "implantation bleeding" which is usually benign and does not mean there is anything wrong. More concerning issues that can present as bleeding include miscarriage, molar or ectopic pregnancy, placenta previa, infection, and placental abruption.
Pregnancy bleeding. Implantation bleeding causing subchorionic hematoms are very common. The placental blood vessels must grow into the uterine blood vessels and leakage can be common. I make the comparison of connecting a pipe to the pipes in a house while the water is still running. You get a leak until the connection is completed.

Related Questions

When do women stop bleeding during pregnancy?

When hormones change. The conceptus ( early pregnancy) begins to create hormones that stop the cycle. This happens soon after the conceptus attaches to the wall of the womb. Those hormones are what turn a pregnancy test positive. Read more...