The hormone build-up to ovulation starts right now in week one of your menstrual cycle. Your pituitary gland, which lies in the base of your brain, produces follicle-stimulating hormone (FSH). During your period, the level of FSH rises steadily, triggering the development of the follicles (around 15-20 each month) in each ovary. As well as containing each egg, the follicles produce estrogen.
The hormone estrogen circulates, affecting the pituitary gland and causing it to produce luteinizing hormone (LH)-this triggers ovulation (see This is Day 14 of your Menstrual Cycle). This week your estrogen levels are low and steady, but will rise dramatically later in your cycle.
Progesterone levels are low during your period, but start to rise several days afterward and stay high for the second part of the cycle. Under the influence of progesterone, the muscles in the cervix relax, easing open the cervical canal. Changes also affect the mucus, which becomes more fluid, so sperm find it easier to swim through. It is progesterone that enables the lining of the uterus to thicken in preparation for implantation of the fertilized egg.