Why Does Implantation Bleeding Happen?

    When ovulation occurs, the hormone progesterone prepares the uterus for
    the implantation of a fertilized egg. This happens every month, even if
    the woman is not trying to conceive. The thickened uterine lining will
    shed each month during the menstrual cycle. This shedding, often
    referred to as a period, will last between three and seven days. If a
    fertilized egg implants in the uterus, implantation bleeding may show
    up, but why does implantation bleeding happen?

    The Uterus, Fertilized Egg and Implantation Bleeding
    After the mature egg is released from the ovary, the corpus luteum increases progesterone levels. Progesterone is a hormone that stays elevated throughout pregnancy making Implantation Bleedingphysical changes to accommodate the growing fetus. The first job of progesterone is to prepare the uterus for implantation. During the three weeks of the month when a woman is not actively ovulating, the uterine lining is thin with a relatively low blood flow. When progesterone levels rise, the uterine lining starts to thicken and blood flow increases.

    At the time of ovulation, the mature egg will move through the fallopian tube toward the uterus. At some point, sperm will meet up with the egg; assuming unprotected sex has occurred at the right time. When the sperm and egg meet-up, the sperm burrows through the thick outer coating of the egg and fertilizes the egg. The blastocyst, as the egg is now called, continues to move through the fallopian tube toward the uterus.

    Progesterone has increased the blood flow to the uterine lining, so it now appears thick and blood-rich. The egg moves into the uterus and snuggles against one of the thicken walls. When the blastocyst meets the uterine lining, it merges with the uterus; implantation. When this merge happens, a small amount of blood may be released. The cervix has yet to close, so the blood passes through the uterus and out of the vagina. This is implantation bleeding.

    Does implantation bleeding hurt?
    In some cases, slight cramping can occur at the time of implantation bleeding.

    Does implantation bleeding always occur?
    Yes, a bit of blood is likely to be released at the time of every implantation, but that does not mean the woman will notice implantation bleeding.

    Does implantation bleeding increase chances of a viable pregnancy?
    No, pregnancy viability does not increase when implantation bleeding occurs. Women who suffer miscarriages have implantation bleeding just like women who carry pregnancies to term.

    How long does implantation bleeding last?
    Implantation bleeding should only last a short time. Typically, women notice a bit of spotting that lasts varying times. Some women may see implantation bleeding just once and other see spotting for a day or two.

    Does implantation bleeding look different than a typical menstrual cycle?
    Yes, implantation bleeding is simply spotting, not actively bleeding. If you are pregnant and actively bleeding, this is not implantation bleeding. Seek medical attention immediately.


    Implantation bleeding is common in all pregnancies. Some women notice the spotting and others never see it at all. Implantation bleeding occurs at the time when the egg implants. Any bleeding after that time should be reported to the attending caregiver. Pregnancy bleeding after implantation bleeding could mean there are complications that need to be addressed.