How does menstrual cycle work?

Women’s menstrual cycle lasts up to 28 days. It starts with the first day of the period (period bleeding is called menorrhea) and ends when the next period begins. It can be divided into two phases: The first phase leads up to ovulation. It prepares an egg to be released from one of the ovaries, and it builds the lining of the uterus. The second phase prepares the uterus to accept the fertilised egg or to start a new cycle if there is no pregnancy. Where no pregnancy takes place, a new period starts, making the beginning of the next menstrual cycle.

The Follicular Phase

The first half of the menstrual cycle leading up to ovulation is called the follicular phase which starts on the first day of period and ends with ovulation. Usually, it takes up the entire first half of our menstrual cycle (10-14 days). The follicular phase is initiated by the hypothalamus. During this phase, the pituitary gland situated below the hypothalamus is prompted to release follicle stimulating hormone (FSH) which instructs the ovaries to prepare an egg (ovum) for ovulation. The levels of estrogen continue to grow and peak just before the ovulation happens. The ovaries are the main source of estrogen in the woman’s body.

While the ovaries are working on producing an egg, the uterus is responding to the estrogen produced by the ovaries by rebuilding the lining that was shed through menorrhea during the last period. The lining of the uterus is called endometrium. It is thinnest during menorrhea, and it thickens throughout the follicular phase until ovulation takes place. If an egg is fertilised, it will implant itself in the lining.

The Luteal Phase

The end of follicular phase is marked by ovulation which is the release of an egg from an ovary. Ovulation normally happens mid-cycle, and it marks the beginning of the second half of the menstrual cycle called the luteal phase. Once ovulation has occurred, the ovary follicle that matured into an egg will transform into a structure called corpus luteum (Latin for yellow body). This will result in a spike in hormone progesterone. If the egg is fertilised, the progesterone from the corpus luteum will support the early pregnancy by allowing the uterus to grow without having contractions and by improving blood flow and oxygen supply.

The lining of the uterus or endometrium releases chemical messengers called prostaglandins which cause the uterus to cramp, causing the period to occur. If a pregnancy occurs, prostaglandin production is inhibited as the progesterone from the corpus luteum will allow the uterus to grow without having contractions. If pregnancy does not occur, corpus luteum stops producing estrogen and progesterone. Prostaglandin production is not inhibited, causing the blood vessels to constrict and the endometrium to break down, causing the period (menorrhea) to start.

The brain, ovaries and uterus work together and communicate through chemical messengers called hormones to keep the menstrual cycle going. If there is a hormonal imbalance or any of the reproductive organs are affected, it will have a negative impact on the menstrual cycle and consequently fertility.