There are various methods of contraception. To better assess this, we will look at the biological processes that occur during the monthly cycle. Basically a woman is capable of her first to her last menstrual to reproduce. This may take more than 40 years. Anyway, the real time during the monthly cycle in which the woman is fertile, is a relatively short event. The cycle starts with the menstruation and ends on the day preceding the next menstruation. For most women, the cycle has 28 days. However, longer or shorter cycles may also occur.

The first phase of the cycle

During the first phase of the cycle an egg grows in the ovary. This process is controlled by the dominant hormone hFSH (follicle stimulating hormone). The protective layer surrounding the egg and the egg itself is called the follicle. It is here that the hormones are produced (estrogens) that trigger the following process: the endometrium begins to develop. At that time, the mucus in the cervical starts to be more liquid. This process takes about 14 days and varies depending on the length of each individual cycle. During this time the estrogenic hormone increases and activates a second hormone. The hLH (luteinizing hormone) causes an ovulation. During the ovulation the follicle ruptures and the egg enters the fallopian tubes. The egg travels several days now in the direction of the uterus. When it meets sperm during his road, and the sperm is fertile, it will reach the uterus. If this does not take place within 12 to 24 hours, the infertile egg will die. Since sperm has a single fertile period of 2-3 days (occasionally up to 5 days), fertilization is possible but only at around 5 to 6 days the middle of the cycle. It is only during this period that the sperm can penetrate the mucous membrane lining the cervix closed.

The second phase of the cycle

Ovulation starts in the second phase of the cycle. Not taking into account whether the egg is fertilized or not, the hLH ensures that the follicle forms a further hormone with the estrogen: progesterone. This is known as the “corpus luteum” or “yellow body” hormone. Why? Because the follicle takes a yellow colour during this phase. Progesterone causes thickening of the endometrium so that the fertilized egg can be implanted. The temperature of the body thereby increases immediately after ovulation to several tenths of a degree. Increasing temperature is an indication that ovulation has occurred. Progesterone is only produced but in this situation.
If fertilization has not occurred, the follicle (yellow body) will stop the increase of progesterone. This means that there is no pregnancy. All those physical preparations for pregnancy have been useless. The body begins to break down the thickened endometrium and the temperature drops back to normal. The following period is underway. The delay of the hormone production stimulates the superior coordination centre to produce hFSH and a new cycle begins.
Finally, it should be said that a woman is only fertile during a few days a Month.  Knowing this is important for the planning of having children and avoiding pregnancy.