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Implantation Timing

    One of the most interesting questions are when does implantation happen exactly? Implantation is when the fertilized egg arrives inside the uterus after having travelled through the Fallopian tube and then attecahes or implants to the lining of the uterus.

    Based on a study published in 1999 by Wilcox, implantation happens on average 9.1 days after ovulation and fertilization with a range of 6-12 days.

    The egg becomes fertilized with a sperm within 12 hours after ovulation, usually in the outer portion of the fallopian tube. It then travels down the fallopian tube, increasing its size along the way. In the fallopian tube it becomes the zygote and doubles to two cells, four cells, then eight cells and becomes the "morula."

    As the morula enters the uterine cavity it's called a "blastocyst." Implantation happens when the fertilized egg (blastocyst) attaches to the uterine wall and starts producing the pregnancy hormone hCG.

    In 84% of pregnancies, however, implantation happened between days 8 and 10 after ovulation. There was an increase of miscarriages when implantation happened later. The rate of early pregnancy loss was 13% when implantation happened by day 9, it rose to 26% with implantation on day 10 after ovulation, it rose to 52% if implantation was on day 11 after ovulation, and to 82% if implantation was later than 11 days after ovulation.

    Literature: Time of Implantation of the Conceptus and Loss of Pregnancy; Allen J. Wilcox, M.D. et.al. NEJM Volume 340:1796-1799