Menstruation is the shedding of the uterine lining. It is normal to the reproductive system of women and happens on a regular basis. Normal menstruation happens around 3 to 7 days. During the monthly menstrual cycle, female sex hormones prepare the uterus to support a pregnancy. If pregnancy happens ( remember do not drink more than one cup of wine a day, over dose of alcohol will drain the vital folic acid that is essential for fetus nourishment. Lack of folic acid may cause birth defect ) menstruation will be suspended until given birth to a child, abortion, or miscarriage. Otherwise, the uterine lining sheds and menstrual blood, tissues, fluid and mucus leave the body through the vagina caused by natural reactions in the female reproductive system due to high levels of certain hormones.
a) Estrogen
Estrogen helps to promote the development and maintenance of female reproductive structures and prepares the follicle for the release of an egg.
b) Progesterone
It helps to prepare the womb lining for the implantation of an egg if pregnancy takes place, milk production occurs.
c) Follicle-stimulating hormone
Produced by the purity gland. Besides regulating the development, growth, maturation and reproductive processes of the female body it also helps to stimulate the growth of immature follicles to maturation resulting in ripening of an egg.
d) Luteinizing hormone
The mature follicles ripen over a period of about seven days and releases more estrogen into the bloodstream stimulating the hypothalamus (a gland in the brain responsible for regulating thirst, hunger, sleep patterns, libido, and other endocrine functions) to release
luteinizing hormones that help to trigger the process in the menstrual cycle by discharging a maturity of follicles most commonly known as an egg resulting in the last stage of the menstrual cycle. Some people may suggest that sex during menstruation will stimulate the production of certain hormone, thereby helping reduce some of the menstrual cramps. According to Chinese herbalist, sex during menstruation not only may cause prolong bleeding and sometime infection in female but also cause the inflammation of your partner reproductive organ and chronic disease later on.

Because of a surge of hormones during the menstrual cycle, some women may experience a variety of sensations before, during, or after their menses such as:
a) Complaints of pain in the inner thighs
b) Bloating and nausea
c) Mood change
d) Constipation
e) Breast tenderness
f) Irritability
g) Stress

And during menstruation:
a) Uterine cramping
b) Abdominal pain
c) Emotional sensitivity
d) Depression

Let's assume the menstrual cycle has a 4 weeks period. If you look closer at the cycle, you can see the 2 and 3 weeks are easier for conception than the others.

I hope this information will help