The endometrium is the name for the mucous membrane constituting the inner layer of the uterine wall, also commonly described as the lining of the womb. Its structure, thickness and state change significantly through the menstrual cycle and its significance to fertility is that this is where the blastocyst will implant if a pregnancy is to progress. In fact “implantation” is a gentle way to describe the process. The ovum (egg) is sometimes described as behaving similarly to a parasite as it adheres to the endometrium. It destroys the epithelium at the point of contact and excavates a cavity into which it imbeds itself after which the endometrium closes over the damaged area. The part of the ovum which causes the damage to the endometrium is called the trophoblast but it doesn’t stop there. It actually absorbs the damaged part of the endometrium, grows over the entire ovum and penetrates into the maternal blood vessels. It continues growing and absorbing and in due course converts the endometrium into the placenta. Fertility doctors have tried to improve implantation rates by inserting embryos under the surface of the endometrium. However this has been found not to have any advantages over the standard practice of placing the embryo in the uterus and letting it “choose” where to implant itself.
Monthly Endometrial Cycles
The endometrium changes through the month along with the menstrual cycle. The whole cycle takes, on average, 28 days. Progress is governed by hormones, as described elsewhere in this website and at the end of the cycle the endometrium is shed and the process starts afresh. The hormones involved, and there are many of them, are similar in all mammals but not all mammals shed their endometrium each month. Have you ever wondered why your cat or dog doesn’t have a period? In fact most mammals reabsorb their endometria in what are called oestrous cycles, rather than our menstrual cycles. One of the aforementioned hormones affecting cycles is melatonin, controlled by the amount of light influencing the pineal gland. This melatonin is then thought to affect the hypothalamus and thus the release of gonadotrophins. Hence one can see the link between the amount of light, i.e. the time of the year, and the reproductive cycle in oestrous-cycle mammals. This is thought to be a survival mechanism, pushing mammals to breed at the most advantageous time of the year so that their offspring can be born in the “right” season. Of course humans, as mammals with our sometimes inconvenient menstrual cycles, do not have this seasonal limitation and thus we can start our reproductive cycles, our sexual activity, in any month of the year. In a fertility treatment cycle, doctors will monitor the endometrium as it grows through the month using ultrasound or, less directly, blood tests to check hormone levels. They can then regulate progress through the administration of medications to affect hormone levels. As the treatment cycle may also have altered hormone levels during the month, which invariably have knock-on effects with other hormones, it is also common to medicate after implantation to ensure the endometrium is not shed.
A Healthy Endometrium
Another function of the endometrium is to prevent the middle layers of the uterus, the myometrium, from adhering to each other. When this happens, the uterus cannot expand and pregnancies will fail or not take place at all. This is more common after trauma or infection (see Asherman’s Syndrome) and is detected by doctors using ultrasound. The endometrium should only grow within the uterus but sometimes parts of it can grow elsewhere in an, often painful, process called endometriosis. Having an effective endometrium/uterus is one of the crucial components, along with a healthy egg and sperm, necessary to produce and carry a baby through to term. As such, it is one of the elements closely monitored with most fertility treatments.