Hallow Dear, During the pregnancy, endometrium not only thickens but even undergoes functional changes which are known as secretory changes. These changes are due to increased levels of hormones Oestrogen (which grows the endometrium) and Progesterone (which makes it secrete). These hormones are secreted in the blood and hence they act all over the body. Thus they are acting even on the endometrium present in the other (right) horn of the uterus even though there is no pregnancy in that horn. You need not be concerned about it so long as your left horn in which pregnancy is implanted grows well to accommodate and nourish the baby. This can be monitored by ultrasonography. I hope this satisfies you. Dr. Nishikant Shrotri