Implantation is when a fertilized egg, which has divided, multiplied, and turned into a cluster of cells called a blastocyst, attaches to the uterine wall. Generally, this happens approximately between six to 12 days after ovulation, and once it does, a pregnancy has officially begun.

Implantation happens when a fertilized egg burrows into the uterine lining and begins to grow. When this can occur for each person is dependent on their overall cycle length, which is not the same for everyone, and when the egg was fertilized.

Generally, ovulation happens about 14 days prior to the beginning of menstruation. An egg is released from either the left or right ovary, after which it will live for about 24 hours unless it is fertilized. If fertilization does not happen, the uterine lining is sloughed off, and menstruation begins.

You don't have to have sex at the exact moment of ovulation—just close to it. In fact, ideally, you will have sex a few times in the days before ovulation occurs. Sperm can live for up to five days inside the body and meet up with an egg making its way through the fallopian tube to the uterus. Sperm then breaks through the barriers surrounding the egg to fertilize it.

The fertilized egg then travels the remaining way to the uterus, a journey that takes several days. Implantation itself usually occurs between six to 12 days after the egg is fertilized.

Sources - https://www.verywellfamily.com/when-does-implantation-occur-in-pregnancy-2371533

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