Menstrual Cycle

A regular menstrual cycle is an important element of successful conception. The menstrual cycle refers to the maturation and release of an egg as well as the preparation of the uterus to receive and nurture the fertilized egg (embryo). The hormones released during the menstrual cycle control the sequence of events that lead to pregnancy. On the first day of the cycle, when menstruation (or your “period”) begins, the uterus sheds its lining from the previous cycle. The typical menstrual cycle lasts for about 28 days and is divided into the following three distinct phases.

During this phase, the hypothalamus and pituitary glands in the brain release a hormone known as follicle stimulating hormone (FSH). FSH stimulates the development of a follicle, which is a tiny fluid-filled sac in each ovary containing a maturing egg. The follicle also secretes estrogen, which produces mid-cycle changes in the cervical mucus. These changes help prepare the cervical mucus to receive and nourish sperm.

The ovulatory phase begins when the level of luteinizing hormone (LH), also release by the pituitary gland, drastically increases or surges. LH causes the follicle to break open and release the mature egg into the fallopian tube. During her reproductive years, a woman usually releases a single mature egg each month. This process is known as ovulation. Cervical mucus is most receptive to sperm around this time and a woman has the best chances of conceiving right before and during ovulation.

It is a common misconception that the ovulatory phase begins around day 14 of your cycle; in fact, it can more easily be determined by 14 days prior to the start of your cycle, which may not be an exact 28 days. Your cycle begins in the first day that you experience regular flow. Once you determine how long your personal cycle lasts, subtract 14 days from the predicted end of the cycle to determine time of ovulation.

During this phase, the follicle that produces the egg becomes a functioning gland called the corpus luteum. The corpus luteum produces progesterone, which prepares the endometrium (lining of the uterus) for the implantation of the fertilized egg.

The ovulatory phase of the menstrual cycle is the optimal time for fertilization. When a couple has intercourse during this time, sperm swim through the cervical mucus, into the uterus and along the fallopian tube, where they meet the egg. Although millions of sperm are released, only one sperm can fertilize an egg. The egg has the capacity to be fertilized for about 24 hours after it is released from the follicle. (If fertilization does not occur, the egg passes through the uterus, and the corpus luteum ceases to function on about day 26. The uterine lining then breaks down and is shed several days later as the next menstrual cycle begins.)

After fertilization, the embryo travels through the fallopian tube toward the uterus. Inside the uterus, the embryo implants itself into the lining on about the 20th day of the cycle and continues to grow into an embryo and eventually a fetus. The corpus luteum continues to produce progesterone to preserve the uterine lining and help maintain pregnancy.