Optimizing implantation. Why does a thin endometrium (uterine lining) result in lower implantation?
For successful implantation to occur an embryo needs a “receptive” uterine lining. In a previous blogs, Katie Koss and Janet Chiarmonte have discussed the various pathologies that can prevent implantation and the three common techniques used to evaluate the uterine cavity. In this blog, the importance of measuring endometrial (lining of the uterine cavity) thickness during cycle monitoring will be discussed.
The endometrium responds to estrogen by growth in its glands and the surrounding tissue (stroma). This is reflected on ultrasound by an increase in thickness and formation of a triple-line endometrial pattern. When ovulation occurs, or when progesterone is started, a “secretory” change occurs in the endometrium, which then becomes receptive for implantation.
What is “normal” endometrial thickness?
There is no definite cut-off level below which implantation will not occur. The consensus seems to be >7 mm in cross-section, with a triple-line endometrial pattern. An endometrial thickness <6 mm is associated with a lower rate of full-term pregnancy. Other publications suggest that thickness >9 mm is important.
- Scar tissue inside the uterine cavity (intrauterine adhesions or Asherman’s syndrome) may result in a thin lining.
- Other causes include medications such as clomiphene citrate, which can cause depletion of endometrial estrogen receptors.
- Prolonged use of birth control pills will also result in a thin uterine lining.
- In a large number of patients, there is no apparent cause for a thin endometrial lining.
Why does a thin endometrium result in lower implantation?
Several theories have been proposed to answer this very important question
- Estrogen receptor abnormalities. For the uterine lining to grow, estrogen has to bind with estrogen receptors. If these receptors are abnormal, the lining will not grow
- Oxygen tension theory.
In a recent publication (Fertil Steril 2011;96:519-521), Dr. Robert Casper from the University of Toronto, Canada offers an interesting mechanism by which a thin lining results in lower implantation. The uterine lining has two layers, a “functional” layer which gets shed with menstruation and a “basal” layer which persists. The functional layer of the endometrium has plenty of small blood vessels (capillaries), in contrast to the larger spiral arteries in the basal layer. With ovulation, there is constriction of the spiral arteries with reduced blood flow to the functional layer. This results in reduced oxygen tension, which is good for embryo implantation. In the endometrium, when the thickness measured by ultrasound is <7 mm, it is the functional layer that is thin or absent, and the implanting embryo would be much closer to the spiral arteries and the higher vascularity and oxygen concentrations of the basal endometrium. The high oxygen concentrations near the basal layer could be detrimental compared with the usual low oxygen tension of the surface endometrium. Interesting!
In my next blog, we will discuss the various treatments that have been tried to correct a thin endometrial lining.