The Primary Organs and Structures in the Reproductive System. The primary structures in the reproductive system are as follows:
- The uterus is a pear-shaped organ located between the bladder and lower intestine. It consists of two parts, the body and the cervix.
- When a woman is not pregnant the body of the uterus is about the size of a fist, with its walls collapsed and flattened against each other. During pregnancy the walls of the uterus are pushed apart as the fetus grows.
- The cervix is the lower portion of the uterus. It has a canal opening into the vagina with an opening called the os, which allows menstrual blood to flow out of the uterus into the vagina.
- Leading off each side of the body of the uterus are two tubes known as the fallopian tubes. Near the end of each tube is an ovary.
- Ovaries are egg-producing organs that hold between 200,000 and 400,000 follicles (from folliculus, meaning "sack" in Latin). These cellular sacks contain the materials needed to produce ripened eggs, or ova.
- The inner lining of the uterus is called the endometrium, and during pregnancy it thickens and becomes enriched with blood vessels to house and support the growing fetus. If pregnancy does not occur, the endometrium is shed as part of the menstrual flow. Menstrual flow also consists of blood and mucus from the cervix and vagina.
|The uterus is a hollow muscular organ located in the female pelvis between the bladder and rectum. The ovaries produce the eggs that travel through the fallopian tubes. Once the egg has left the ovary it can be fertilized and implant itself in the lining of the uterus. The main function of the uterus is to nourish the developing fetus prior to birth.|
Reproductive Hormones. The hypothalamus (an area in the brain) and the pituitary gland regulate the reproductive hormones. The pituitary gland is often referred to as the master gland because of its important role in many vital functions, many of which require hormones. In women, six key hormones serve as chemical messengers that regulate the reproductive system:
|Click the icon to see an image of the hypothalamus and pituitary gland.|
- The hypothalamus first releases the gonadotropin-releasing hormone (GnRH).
- This chemical, in turn, stimulates the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
- Estrogen, progesterone, and the male hormone testosterone are secreted by the ovaries at the command of FSH and LH and complete the hormonal group necessary for reproductive health.
|Click the icon to see an image of the pituitary gland.|
Ovulation. The process leading to fertility is very intricate. It depends on the healthy interaction of two sets of organs and hormone systems in both the male and female. In addition, reproduction is limited by the phases of female fertility. Nevertheless, this astonishing process results in conception within a year for about 80% of couples. Only 15% conceive within a month of their first attempts, however, and about 60% succeed after six months.
A woman's ability to produce children occurs after she enters puberty and begins to menstruate. The process to conception is complex:
- With the start of each menstrual cycle, follicle-stimulating hormone (FSH) stimulates several follicles to mature over a two-week period until their eggs nearly triple in size. Only one follicle becomes dominant, however, during a cycle.
- FSH signals this dominant follicle to produce estrogen, which enters the bloodstream and reaches the uterus. There, estrogen stimulates the cells in the uterine lining to reproduce, therefore thickening the walls.
- Estrogen levels reach their peak around the 14th day of the cycle (counting days beginning with the first day of a period). At that time, they trigger a surge of luteinizing hormone (LH).
LH serves two important roles:
- First, the LH surge around the 14th cycle day stimulates ovulation. It does this by causing the dominant follicle to burst and release its egg into one of the two fallopian tubes. Once in the fallopian tube, the egg is in place for fertilization.
- Next, LH causes the ruptured follicle to develop into the corpus luteum. The corpus luteum provides a source of estrogen and progesterone during pregnancy.
|Click the icon to see an image of the corpus luteum.|
Fertilization. The so-called "fertile window" is six days long and starts five days before ovulation and ends the day of ovulation. Fertilization occurs as follows:
- The sperm can survive for up to three days once it enters the fallopian tube. The egg survives 12 to 24 hours unless it is fertilized by a sperm.
- If the egg is fertilized, about two to four days later it moves from the fallopian tube into the uterus where it is implanted in the uterine lining and begins its nine-month incubation.
- The placenta forms at the site of the implantation. The placenta is a thick blanket of blood vessels that nourishes the fertilized egg as it develops.
- The corpus luteum (the yellow tissue formed from the ruptured follicle) continues to produce estrogen and progesterone during pregnancy.
If the egg is not fertilized, the corpus luteum degenerates into a form called the corpus albicans, and estrogen and progesterone levels drop. Finally, the endometrial lining sloughs off and is shed during menstruation.
Typical Menstrual Cycle
Typical No. of Days
Follicular (Proliferative) Phase
Cycle Days 1 through 6: Beginning of menstruation to end of blood flow.
Estrogen and progesterone start out at their lowest levels.
FSH levels rise to stimulate maturity of follicles. Ovaries start producing estrogen and levels rise, while progesterone remains low.
Cycle Days 7 - 13: The endometrium (the inner lining of the uterus) thickens to prepare for the egg implantation.
Cycle Day 14:
Surge in LH. Largest follicle bursts and releases egg into fallopian tube.
Luteal (Secretory) Phase, also known as the Premenstrual Phase
Cycle Days 15 28:
Ruptured follicle develops into corpus luteum, which produces progesterone. Progesterone and estrogen stimulate blanket of blood vessels to prepare for egg implantation.
If fertilization occurs:
Fertilized egg attaches to blanket of blood vessels that supplies nutrients for the developing placenta. Corpus luteum continues to produce estrogen and progesterone.
If fertilization does not occur:
Corpus luteum deteriorates. Estrogen and progesterone levels drop. The blood vessel lining sloughs off and menstruation begins.
Stages and Features of Menstruation
Onset of Menstruation (Menarche). Previous evidence had set the onset of menstruation, called the menarche, at an average of age 12 or 13. Recent studies, however, set the time of onset earlier by about one year in Caucasian girls and two years in African American girls. Currently, the youngest possible age for normal puberty is 7 years old for Caucasians and 6 years old for African Americans, down from a previous low of 8 years for both.
Evidence is pointing to the increasing incidence of childhood obesity as a major cause of the trend in earlier menarche onset. (Obesity is also highly associated with hormonal disorders in girls entering puberty at young ages.) Environmental estrogens found in chemicals and pesticides are also suspects.
Length of Monthly Cycle. The menstrual cycle can be very irregular for the first one or two years, usually being longer than the average of 28 days. The length then generally stabilizes to an average of 28 days, although the cycle length may range from 20 to 45 days and still be considered normal. A variation of 10 days or more--either more or fewer days--may have an impact on fertility, however. When a woman reaches her 40s the cycle lengthens, reaching an average of 31 days by age 49. A number of factors can affect cycle length at any age.
Risk Factors for Shorter Cycles
Risk Factors for Longer Cycles
Regular alcohol use.
Being under 21 and over 44.
Being very thin (also at risk for short bleeding periods).
Competitive athletics (also at risk for short bleeding periods).
Length of Periods. Periods average 6.6 days in young girls. By the age of 21, menstrual bleeding averages six days until women approach menopause. It should be noted, however, that about 5% of healthy women menstruate less than four days and 5% menstruate more than eight days.
Normal Absence of Menstruation. Normal absence of periods can occur in any woman under the following circumstances:
- Menstruation stops during the duration of pregnancy. Some women continue to have irregular bleeding during the first trimester. This bleeding may indicate a threatened miscarriage and requires immediate attention by the physician.
- When women breastfeed they are unlikely to ovulate. After that time, menstruation usually resumes and they are fertile again.
- Perimenopause starts when the intervals between periods begin to lengthen, and it ends with menopause itself (the complete cessation of menstruation). Menopause usually occurs at about age 51, although smokers often go through menopause earlier.