According to Krissi Danielsson for About.com, implantation bleeding typically manifests as light spotting, not as heavy blood flow with clotting. Not all women experience implantation bleeding, which is believed to be caused by the fertilized egg becoming implanted in the uterus.
HealthTap points out that bleeding with large clots is more likely to be caused by a period or a miscarriage than by implantation bleeding. Cramping or passing clots of blood is not a good sign after pregnancy begins, and should be checked by an obstetrician or gynecologist. While very small clots sometimes appear as part of implantation bleeding, they are rare. Implantation bleeding is sometimes mistaken for a very light period by women who do not yet realize they are pregnant.