The inner lining of the uterus is called the endometrium. The endometrium is the tissue to which a fertilized egg, or blastocycst, attaches for the duration of a pregnancy. The endometrial size varies throughout the menstrual cycle, but should be less than 8 mm in post-menopausal women.
The menstrual cycle lasts approximately one month. During the cycle, a follicle matures and then it ruptures to release an egg that is swept into the fallopian tube for fertilization. The endometrium thickens throughout the menstrual cycle in preparation for egg implantation. If implantation does not occur, the endometrium is expelled, a process called menstruation.
The endometrium starts off as a thickened lining, sometimes measuring up to 15 mm in thickness. Menstruation occurs during the first week of the menstrual cycle. As the endometrium is expelled, it may vary in thickness and may have an irregular appearance on ultrasound. As the week progresses, the endometrium becomes thinner. By then end of menstruation, the endometrium should measure approximately 2 to 3 mm thick.
During the proliferative phase, a follicle begins to grow on one of the ovaries. While this follicle grows, it secretes estradiol, an estrogen that is responsible for the thickening of the endometrium prior to ovulation. Fertility increases during this stage of the cycle. During the proliferative stage, the endometrium can become as much as 8 mm thick. On ultrasound, it has the appearance of three lines.
Ovulation marks the end of the proliferative phase and the beginning of the secretory phase. The mature follicle ruptures and releases an egg, called an ovum. This is called ovulation. Fertility is greatest between five days prior to ovulation and two days after. Some fertility clinics like to see a 1-cm thick endometrium prior to ovulation.
The follicle that ruptured to release the egg is now called the corpus luteum. The corpus luteum releases progesterone. Progesterone is responsible for the further thickening of the endometrium in preparation for blastocycst implantation. During the secretory phase, the endometrium can thicken to as much as 15 mm.