Implantation bleeding vs period

Implantation bleeding, explained

Implantation bleeding is light bleeding that you might experience if a fertilized egg implants in the lining of your uterus.

According to the American College of Obstetricians and GynecologistsOpens a new window (ACOG), this can happen 1 to 2 weeks after fertilization.

Here's why experts think it happens:

After the sperm fertilizes the egg, it starts the three- or four-day trip from the fallopian tube to your uterus.

During this time, it divides into hundreds of identical cells. Meanwhile, your ovaries release estrogen that causes the lining of the uterus (the endometrium) to thicken and develop more blood vessels.

When does implantation bleeding occur

The uterine lining is preparing to host the quickly growing embryo.

Once the embryo enters the uterus, it's called a blastocyst, a tiny ball of several hundred cells. A day or two later, it begins burrowing into the blood-rich lining of your uterus, where it continues to grow and divide.

As it burrows into the endometrium, the blastocyst may cause littl

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