Menstruation. Artwork of a target centred on the uterus (red) during menstruation. Blood is flowing down the uterus towards the vagina as its lining (endometrium, pale pink) breaks down. The ovaries (yellow) and the fallopian tubes (pink) are at upper left and right. The bones of the pelvis are brown and white. The endometrium thickens during both the proliferative phase of the menstrual cycle, when the egg (ovum) ripens, and the secretory phase, when the egg is released from an ovary. If the egg is not fertilized, menstruation occurs. Heavy bleeding (menorrhagia) may occur due to a hormonal imbalance, fibroids, polyps or the presence of an intrauterine device (IUD).