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The monthly fertility pattern which occurs regularly in most women beginning at puberty and ending at menopause is called the menstrual cycle. Each cycle is divided into two parts --- before ovulation and after ovulation. Over a 28-day period the cycle typically follows this pattern:
Part 1. Day 1 of your cycle is the day menstruation (or bleeding) begins and usually lasts from 3-5 days. This first part, from menstruation to ovulation, may vary from 13 to 20 days in length and is referred to as the follicular phase of the cycle, as this is when the follicle develops the mature egg. The actual length may not only differ from one woman to another but also differs in some women from month to month. It is during this critical first part of the cycle that fertilization can occur.
The regularity of this pattern may be upset by such common occurrences as sickness, stress, physical exertion or even climate changes. A rise in the level of Luteinizing Hormone (LH) accompanies a change in the cervical mucus in anticipation of the follicle rupturing and releasing a mature egg towards the end of this first half of the cycle. (The change in cervical mucus allows sperm in the vagina access to the uterus and fallopian tubes.) This surge in hormone level can be detected in your urine and is commonly used to predict ovulation and your fertile period. The ruptured follicle becomes the corpus luteum and produces hormones that effect the rest of the cycle.
Part 2. The second part of the cycle, from ovulation to menstruation, is about the same length in all women, and is called the luteal phase. It is during this part that the egg travels down the fallopian tube toward the uterus --- an event which consistently occurs 14 to 16 days before the onset of menstruation regardless of the length of a woman's menstrual cycle. If a single male sperm unites with the egg while it is in the tube the fertilized egg may attach to the spongy lining of the uterus. Pregnancy begins if the fertilized egg implants into the uterine lining . If fertilization does not take place the egg will break apart in a day or two. At about Day 25, hormone levels begin to drop. This causes the lining of the uterus to break down and in a few days it is shed in a menstrual period. Another cycle has begun.
For access to educational videos produced through PBS explaining human reproduction, and the role of sperm and egg, go to the following link PBS MOVIES