The first half of the proliferative phase starts around day 6 to 14 of a person's cycle, or the time between the end of one menstrual cycle, when bleeding stops, and before ovulation. At this phase, the endometrium begins to thicken and may measure between 5–7 mm.

Does endometrium thicken before period?

In a typical menstrual cycle, estrogen made by the maturing ovarian follicle causes the glands to grow and the endometrium to thicken (partly through an increased blood supply). This thickening of the uterine lining is called the proliferative phase of the menstrual cycle.

What should be the endometrial thickness on day 13?

In the women with a menstrual cycle length of 28–30 days, the ET was 7 mm on days 1–6, but it increased from 5.4 mm immediately after menstruation (day 7 or 8) to 9.2 mm on days 13–14.