Bleeding around the time of an expected period isn't unusual, even if a woman is newly pregnant. Bleeding in very early pregnancy can have several causes, some normal and harmless, others more concerning. It can be difficult to determine the exact cause of bleeding in very early pregnancy; an ultrasound around four to five weeks can determine that the pregnancy is implanted in the uterus and is progressing normally.
Spotting around the time of implantation, when the growing embryo burrows into the lining of the uterus, is common and harmless. Implantation generally happens around six to 12 days after conception, states the American Pregnancy Association (APA). Conception occurs approximately two weeks before the expected time of the menstrual period. Implantation bleeding is usually very light, scant and lasts for only a few days.
Chemical pregnancies occur when an egg is fertilized and starts to implant, but then stops growing. A period that may be heavier than usual then begins around the time a period would normally begin. The most common cause of chemical pregnancy is chromosomal abnormalities in either the egg or sperm. Chemical pregnancies occur frequently; Norman Ravski, M.D. of Yale-New Haven Hospital, states on HealthLink.com that around 30 to 50 percent of early pregnancies are chemical pregnancies. Unless a woman does a very early pregnancy test, she will not realize that she was pregnant.
Ectopic pregnancies implant outside the uterus, most often in the fallopian tubes but possibly in the ovary, cervix or abdominal cavity. Because the hormone levels are normally lower in an ectopic pregnancy, a small amount of light vaginal spotting may occur around the time of an expected period. As the embryo continues to grow, symptoms become more pronounced, with heavier bleeding and abdominal pain. Ultrasound can show whether the pregnancy is in the uterus as early as four weeks, around the time of a missed period, reports the Mayo Clinic.