Why does the uterine wall become thinner as pregnancy progresses?

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Close-up of a home pregnancy test device showing a blue plus sign in the result window, indicating a positive result for pregnancy. — pregnancy Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.

The uterine wall becomes thinner as pregnancy progresses primarily because it is stretched by the growing conceptus and the amniotic fluid that surrounds it.1

While the uterus undergoes significant growth during pregnancy—including an increase in the number of muscle fibers and the size of individual muscle cells—this expansion is necessary to accommodate the developing fetus. As the contents of the uterus demand more space, the uterine wall is stretched and thinned, eventually converting the organ into a large, elastic, fluid-filled cylinder by the time of birth.1