One example of common, harmless blood loss early on in a pregnancy is through the implantation of the fertilized egg into the uterus wall.
Abnormalities in the cervix, such as a polyps or inflammation (Chlamydia, for example) can also cause blood loss without affecting the pregnancy. This type of blood loss often occurs after intercourse or after (hard) bowel movements. A rare cause of blood loss is an ectopic pregnancy.
Blood loss can occur in the first 16 weeks of pregnancy. In half of these cases blood loss stops after a few days and has no significance. For the other half, blood loss may indicate an incipient miscarriage.