Introduction:

The Reproductive System

• 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 200,000 - 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 and a woman starts her menstrual flow (or "period"). 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 control the reproductive hormones. In women, six hormones help regulate the reproductive system:

• Gonadotropin-releasing hormone (GnRH) is released by the hypothalamus.
• GnRH 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.

Ovulation

Ovulation is the process where a mature egg (ovum) is released from the ovary. The egg begins its development inside a follicle of the ovary:

• With the start of each menstrual cycle, 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 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.

Fertilization

The so-called "fertile window" is 6 days long and starts 5 days before ovulation and ends the day of ovulation. Fertilization occurs as follows:

• The sperm can generally survive for up to 5 days once it enters the fallopian tube. The egg survives 12 - 24 hours unless it is fertilized by a sperm.
• If the egg is fertilized, it travels 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.

Stages and Features of Menstruation

What is Menstruation? Menstruation, also called a "period," is the cyclical flow of blood from the uterus in women between puberty and menopause.

Onset of Menstruation (Menarche). The onset of menstruation, called the menarche, typically begins between the ages of 12 - 13 years. Menarche generally occurs 2 - 3 years after initial breast development (breast budding). African-American and Hispanic girls tend to mature slightly earlier than Caucasian girls. A higher body mass index (BMI) during childhood is associated with an earlier onset of puberty. Environmental factors and nutrition may also affect menarche timing.

Length of Monthly Cycle. The menstrual cycle can be very irregular during the first 1 - 2 years, ranging from 21 - 45 days. The length then generally stabilizes to an average of 28 days, although the cycle length may range from 21 - 35 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. The cycle lengthens when a woman is in her 40s, reaching an average of 31 days by age 49. A number of factors can affect cycle length at any age.

Length of Periods. Periods average 6.6 days in adolescent girls. By the age of 21, menstrual bleeding averages 6 days until women approach menopause. However, about 5% of healthy women menstruate fewer than 4 days and 5% menstruate more than 8 days.

Normal Absence of Menstruation. Normal absence of periods can occur in any woman under the following circumstances:

• Menstruation stops during 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 doctor.
• When women breast-feed 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.