Your periods is what usually tells you that you are pregnant. Periods are one of the most important things that happens to a woman. Different factors can influence the period. Being overweight, underweight and diet all affect the periods.
It is necessary to remember the date of your period; so that you also know that it is regular. When you are planning to have a baby; missing a period will tell you if you may be pregnant.
Also you need to know the date your period started so that you can try to conceive according to the most fertile dates and that can be calculated only if you remember the date of the period.
If you are unable to conceive the doctor will first ask you the pattern of your periods. So once you begin trying to conceive try to keep a diary and keep track on the dates that you began your menses.
Even if you do get pregnant it is again your period date that your doctor would need to know to tell you how much in to the pregnancy you are and also your approximate due date. The ideal cycle is a 28 day cycle and the first day of your period is calculated as day one and the most fertile time and best time to have sex if you want to conceive is on the fourteenth day calculated from the first day of the period.
If you are having irregular periods and it is not a good 28 day cycle you will need to still write the dates so that the doctor can understand the pattern of your cycle. Also keep track of the amount of flow that you have during your period. Inform your doctor of any medication that you are having as that also may be interfering with your cycle.
Once a girl reaches puberty it is advisable to ask her also to keep track of her periods and her flow so that if there is any problem it can be treated immediately as all this may affect the chances of conceiving later on. Since children at a young age do not realize its importance the mother can maintain a diary and ask the child about the flow and keep a track.