Endometrium… There are a lot of things that happen before you become pregnant. First, an egg is released from the follicle during ovulation. It passes into the fallopian tube, where it hopefully is fertilized. From there, it passes into the uterus and implants, and there it is nourished and grows over the following months during a pregnancy. But while a lot of discussion goes into the first and last stages of this journey to pregnancy, not as much discussion goes into asking: where does an egg land in the uterus?

The answer is: a woman’s uterine lining. The uterine lining is the layer that swells in anticipation of the fertilized egg. And while it doesn’t get as much attention as ovulation, it has an important role to play in the journey when trying for a baby.

Form And Function of the Endometrium

The uterine lining – also known as the endometrium – is composed of two layers. The functional layer builds up after the end of the previous menstrual cycle, swelling and forming a perfect environment for a fertilized egg. In the absence of an egg, the functional layer will eventually be shed during menstruation. The basal layer, which forms the functional layer, is never shed – it remains behind to start the process over again.

When an egg is fertilized, however, it implants in the uterine lining. Implantation occurs between 6 and 12 days after ovulation, with most occurring on days 8 to 10. Typically, this will be about 5 to 6 days after fertilization. The embryo then 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. The uterine lining is now known as decidua, which develops into the placenta during the duration of a pregnancy.

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.

Role In Implantation

Studies show that cycles with thicker uterine linings result in successful pregnancies more often. A triple-line endometrium is also considered favorable. This is when the lining shows a lighter line on an ultrasound between two darker lines, indicating separation between the basal and functional layers.

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.

It is good to understand the uterine lining and its role in fertility. If you have concerns about your endometrial health, talk to your doctor to determine what may be behind any implantation difficulties. In the meantime, if you are looking for more information on fertility and tips for becoming pregnant, follow us on FacebookTwitter, and Instagram or sign up for our newsletter.