During the reproductive phase, a women's body undergoes many hormonal changes. One of them is the menstrual cycle. During the development of the egg menstrual cycle takes place which results in the shedding of the endometrial lining. It is a 28 days cycle, if the egg is not fertilized women get periods at the end of this cycle.
This cycle has four phases which include the menstrual phase, follicular, ovulation, and luteal phase. The length of the menstrual normally varies from woman to woman. Generally, they are 28 days cycles, but for some women, it lasts from 21 days to 40 days. Our body gets pregnant during this cycle. If fertilization occurs during this cycle, the fertilized egg known as a zygote travels through the fallopian tube and gets implanted in the uterus which results in pregnancy.

Women get pregnant during the ovulation period (ovulation phase). Ovulation happens during the fertile days of women. If it is a 28 days cycle, the 12 to 14 days after the start of the period are considered as the fertile days. If proper ovulation does not happen then the women may have a polycystic ovarian syndrome. So if you are not getting periods properly, then consult a doctor and get medical help.