Which hormone is primarily responsible for growth of the endometrium after ovulation?

The hormone primarily responsible for the growth of the endometrium after ovulation is progesterone.

Here are the steps that explain this process:

1. The menstrual cycle begins with menstruation (day 1), which is the result of a drop in both estrogen and progesterone, the two main hormones the ovaries produce.

2. After menstruation, the body begins the follicular phase (day 1-13 on average), where the pituitary gland releases follicle-stimulating hormone (FSH). This hormone stimulates the ovary to produce around five to 20 follicles (tiny nodules or cysts), which bead on the surface.

3. Each follicle houses an immature egg. Usually, only one follicle will mature into an egg, while the others die. The growth of the follicles stimulates the lining of the uterus (the endometrium) to thicken in preparation for possible pregnancy, this is mainly due to the increase in estrogen produced by the follicles.

4. Ovulation occurs around day 14 of the cycle, in response to a surge in luteinising hormone (LH) and to a lesser degree FSH. This is when the mature follicle bursts and releases its egg.

5. After ovulation, the ruptured follicle forms a corpus luteum. The corpus luteum produces progesterone. Progesterone further prepares the endometrium for possible pregnancy by causing it to thicken and become more vascularised.

6. If pregnancy does not occur, the corpus luteum withers and dies, usually around day 22 in a 28 day cycle. Progesterone levels drop, which causes the lining of the uterus to shed, known as menstruation.

7. If pregnancy occurs, the corpus luteum keeps producing progesterone until the placenta is developed enough to produce its own hormones.

So, in summary, progesterone is the hormone primarily responsible for the growth of the endometrium after ovulation.