The formation of corpus luteum cysts during pregnancy is as follows. After the egg is expelled from the ovary, the follicular fluid flows out, the pressure in the follicular cavity decreases, the follicular wall collapses and is surrounded by the outer follicular lining of connective tissue, which together form the corpus luteum. If the egg is fertilized, the corpus luteum enlarges under the action of human chorionic gonadotropin and transforms into the corpus luteum of pregnancy, which secretes estrogen and progesterone to maintain pregnancy and is 2-3 cm in size. If there is a large amount of fluid inside the corpus luteum, it can make the corpus luteum of pregnancy larger than 3cm, called luteal cyst of pregnancy, which is a normal physiological phenomenon, do not be nervous and avoid strenuous exercise, which may cause luteal cyst tip torsion. In the 3rd trimester luteal cysts gradually shrink or disappear, the placenta forms to replace the role of the corpus luteum and continue the pregnancy.