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The menstrual cycle involves rhythmical changes in the ovaries and uterus that prepare the female for pregnancy. The cycle lasts an average of 28 days. It is regulated by four hormones—follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogens and progesterone.
月经周期是（因为）卵巢和子宫为（女性）准备受孕而发生的节律性改变。这个周期持续平均28天。它由四种激素调节：卵泡刺激素，黄体生成素，雌激素和孕激素。
The first day of menstruation marks the first day of the menstrual cycle. It begins with the shedding of the uterine endometrium. The tiny capillaries of the endometrium rupture and blood together with dead endometrial cells is discharged through the vagina. Then a follicle with an egg cell inside it gradually matures in the ovary. The uterine endometrium begins to build up again. It gradually becomes thicker and softer.
月经的第一天标志着月经周期的开始，它以子宫内膜流血开始。子宫内膜的小毛细血管破裂，血和死亡的内膜细胞一起通过阴道排出。然后内有卵细胞的卵泡开始在卵巢内逐渐发育成熟。子宫内膜开始重新建立，变得越来越厚，越来越软。
