The thickness of the endometrium normally changes throughout the menstrual cycle. During menstruation, the endometrium is shed. Under the influence of the hormone estradiol, the endometrium then regenerates and usually develops to a normal thickness of 8 or more millimeters (mm). When a woman is undergoing infertility treatment, the thickness of her endometrium is regularly measured using ultrasonography.
When the endometrium fails to develop to at least 8 mm, the embryo may fail to implant because of endometrial immaturity or dyssynchrony. Although this problem is not very common, when it occurs, it can be difficult to correct. Typical treatments consist of providing additional estrogen early in the menstrual cycle or altering the timing of progesterone administration. Other therapeutic agents include small doses of aspirin (80 to 100 mg per day), and some physicians have prescribed sildenafil (Viagra) vaginal suppositories for their female patients with variable success. Some women cannot achieve pregnancy in a fresh IVF embryo transfer cycle but readily become pregnant when the embryos are transferred in a frozen–thawed nonstimulated treatment cycle. On rare occasions, a couple may need to use a gestational carrier to successfully overcome abnormalities involving the endometrium and implantation.
Of course, failure of the endometrium to achieve a minimum thickness of 8 mm does not necessarily translate into a problem with the endometrium. In our practice, we have seen many patients with maximal endometrial development of only 4 to 7 mm successfully achieve pregnancy, including delivery of twins. A variety of testing methods to assess endometrial maturity have been proposed, including endometrial biopsy testing for surface proteins called integrins, but such testing remains somewhat controversial in terms of its predictive value.