According to Mayo Clinic, implantation bleeding is generally very light and lasts a very short time. Occurring sometime between 10 and 14 days after the fertilization of an egg, implantation bleeding happens around the same time as a missed menstrual period, and in some cases, it may be mistaken for a period.
According to What to Expect, the blood from implantation bleeding is often darker than typical menstrual blood, but it can also be pink or red. One-third of women report having implantation bleeding. Implantation bleeding can also be accompanied by a light, mild cramping similar to regular menstrual cramps. Because this sign of pregnancy occurs before a home pregnancy test registers a positive result, taking a pregnancy test at this time is often inconclusive.
Implantation bleeding occurs when the egg attaches itself to the uterine wall, according to What to Expect. While implantation bleeding is completely normal, not all types of bleeding are normal. Bleeding can also indicate problems in a pregnancy, such as a molar pregnancy or ectopic pregnancy. If vaginal bleeding lasts longer than a couple of days and consists of more blood than light spotting, it is important to contact a doctor to determine the cause.