Answer:
Endometrium / Uterine lining
Step-by-step explanation:
The change in the production of the hormones Estrogen and Progesterone dictate a female's menstrual cycle. For about three weeks, the endometrium which lines the inside of the uterus thickens. If that female becomes pregnant, the endometrium stays as it's actually what a fetus uses to grow, like the yolk in an egg. If the female does not become pregnant, the uterine lining is shed. Most mammals simply absorb it back into the body, but in humans, the uterus contracts and sloughs it off, sending it through the vaginal canal and out of the body, a process which takes about a week.
The red boxes aren't labeled, but it's the topmost.