At the beginning of each cycle, due to the hormone FSH (follicle stimulating hormone), about 15 to 20 Graafian follicles containing egg cells start to mature in both ovaries. The follicles produce the hormone estrogen, which is necessary for ovulation. Ovulation occurs when the largest of the follicles (usually only one, and usually about 18 to 22 mm in size) releases an egg from one of the two ovaries. This usually takes around 14 days from the start of the cycle, but can take as few as eight days or as long as a month after. The time it takes for the egg cell to mature and for the body to prepare for ovulation depends mainly on how fast estrogen levels rise. A sufficient level of estrogen releases the hormone LH, which releases the egg from the follicle (ovulation occurs) usually one or two days after it has reached its peak level. A released egg can be fertilized for around 12 to 18 hours after ovulation.
During that time, the cracked, empty follicle dies and changes into the corpus luteum, which starts to release another hormone called progesterone. The corpus luteum lives for about 10 to 16 days, or 12 days on average. This is how long it takes from ovulation until the next menstruation (the so-called “luteal phase”). If the luteal phase is shorter than 10 days, and a couple is trying to conceive, it may indicate a problem (read more: What is a luteal phase and what does it mean when it is short?) Progesterone, which is released by the corpus luteum, plays an important role in helping the fertilized egg to implant itself (it influences the quality of the endometrium.) Progesterone also prevents the release of additional eggs (occasionally more than one egg gets released during the same cycle; usually a maximum of 24 hours after the release of the first egg), the cervical mucus dries out, and the cervix lowers its position and hardens.