The uterus has 2 basic parts. One is the muscle, also called the myometrium. This gives the uterus its strength, even when stretched thin during pregnancy. Blood travels from the big arteries of the pelvis, through the smaller arteries of myometrium, and during pregnancy from the myometrium to the placenta. Fibroids start to grow from the myometrium.
The other part is the endometrium. This is the layer of glands that line the inside of the uterus. Almost all of the endometrium sheds during menstruation, and then the new glands arise from the small numbers of glands that were left behind. The glands support the pregnancy.
Because the myometrium and endometrium are different tissue types, and of different densities, the two can be seen distinctly via ultrasound.
The ultrasound view of the endometrium changes throughout the cycle. During the period, the endometrium can look very different from day to day and from person to person. By day 2, some women have bled a lot and have shed most of the glands, and therefore their linings look very thin on ultrasound. Some women have not bled much, so most of the lining has not yet shed, and their linings look thicker. This is why I never comment on the endometrium on a day 2 or day 3 scan. If it’s thick it will thin out, if it is thin, it will get thicker as days go by. This is why when some of you ask at the baseline scan, “how does the lining look”, I always say it doesn’t matter how it looks.
The glands then grow thicker day by day and this is the result of increasing exposure to estrogen, which is coming from the growing follicle. This growth phase is also very different from person to person. Some women need very little estrogen to get the lining thicker, and they do not need the estrogen levels to increase with time. All they need is a small amount of estrogen to get the lining to grow, and it will get thicker by itself. In other women, more estrogen is needed. For some women, the thickness can be related to the estrogen level, i.e. the lining can grow to a certain thickness, not more unless the estrogen levels get higher.
The lining stops getting thicker after ovulation, at the start of progesterone production from the follicle, now called the corpus luteum. Progesterone does not make the lining thicker. Progesterone changes the cells of the glands so that they can allow the embryo to grow. Usually the lining stays the same, or it may even get a little thinner after ovulation. Over the next 2 weeks, the lining is undergoing considerable change from day to day, but we can’t see any of these changes on ultrasound.
We can however see a basic change between the pre and post ovulation lining. Before ovulation, the glands look darker, and we can many times see the “triple” pattern, also called the ring. This is shown in the ultraound at the top of the page After ovulation, the emdometrium gets a little brighter, and there is no longer a ring, the pattern is more homogenous.