There are a lot of things that need to happen before you can become pregnant. First, an egg is released from a follicle during ovulation. It passes into the fallopian tube, to await fertilization. If the egg does become fertilized (fingers crossed), it passes into the uterus and implants, where it is nourished and grows over the course of a pregnancy. One key part of the female body that makes all this possible is the endometrium.

The endometrium is the mucous membrane that lines the uterus. It thickens during the menstrual cycle as preparation for the possible implantation of an embryo (aka a fertilized egg). Implantation and the endometrium go hand in hand. Here’s what you should know.


What is the endometrium, and what does the endometrium do? 

The uterine lining — also known as the endometrium — is composed of two layers:

  • The basal layer never sheds. It is the part of the endometrium that helps form the other layer.
  • The functional layer builds up after the previous menstrual cycle ends, forming a perfect environment for a fertilized egg. If the egg is not fertilized, the functional layer will shed during the next menstrual cycle.

When an egg is fertilized, it implants in the uterine lining. This usually occurs between 6 and 12 days after ovulation and about 5 to 6 days after fertilization.

Once implanted, the embryo secretes hormones, including human chorionic gonadotropin (hCG), which signals to the mother’s body that she is pregnant. This halts the menstrual cycle and prevents the functional layer of the endometrium from being shed.

From this point on, the uterine lining will turn into the placenta, which provides oxygen and nutrients to the baby.

Even when there’s no implanted egg to sustain, the uterine lining serves a critical function. The endometrium prevents adhesions of the underlying myometrium layer, which contains the muscle tissue that causes contractions. If adhesions form, it’s possible for the uterus to collapse, preventing normal menstruation and pregnancy.

What role does the endometrium play in implantation?

Studies show that cycles with thicker uterine linings result in successful pregnancies more often. In most successful implantations, the arrival of the egg coincides with peak levels of luteinizing hormone (LH), which primes the endometrium to accept the embryo.

In addition to LH, a chemical called trypsin, produced by the embryo signals the uterine lining to prepare for implantation. A favorable endometrium is thick and exhibits a triple-line pattern under ultrasound imaging.

Unfortunately, even if an egg is successfully fertilized, it is possible for the embryo to fail to implant in the uterine lining. In two out of three cases, this is due to inadequate uterine receptivity. Many factors can contribute to uterine receptivity, including hormones and a type of proteins called cytokines, which allow cells to send and receive signals. Some medications, including progestins and progesterone, can increase the chances of successful implantation after repeated failure.

Want to learn more about trying to conceive? Check out these posts:

Ovulation Calculator

Hormones and Fertility

Male Hormones and Fertility