Human Reproduction

In order to have an appreciation for and an understanding of Natural Family Planning, (NFP) it is helpful to have some background in human (male and female) reproductive anatomy and physiology. To begin, you should know that the male cell of reproduction is called the sperm, and the female cell of reproduction is called the egg or ovum. The uniting of these two cells is called conception.  It is the beginning of human life.

The male seeds or sperm are produced in the two male gonads or testes. Besides producing sperm, the testes also produce and secrete the male sex hormone called testosterone. Other male reproductive structures include the ducts and glands that secrete fluids which nourish and protect the sperm. The penis introduces millions of sperm into the vagina of the female at the time of intercourse. However, there is only one sperm among millions that will eventually fertilize an egg and result in a new human individual. Sperm can live for 3-5 days when in a good environment.  

The female eggs or ova are stored in the two acorn sized structures called the ovaries. The ovaries produce the female gametes (ova) and also secrete female sex hormones, estrogen and progesterone. The two ovaries lie within the pelvic cavity (see Figure 1). Other female reproductive structures include the two oviducts (also called fallopian tubes), which carry the sperm to meet the egg, and the uterus (or womb) where development of the baby occurs. The uterus is continuous with the vagina, which receives sperm from the male and serves as the lower part of the birth canal. The base and opening of the uterus is called the cervix. The canal that travels from the vagina through the cervix and into the womb is called the endo-cervical canal, and this canal is lined with special mucus producing cells.


Figure 1: Internal Female Reproductive Anatomy


In order for sperm to survive, they need to be in a good environment. The woman’s vagina is acidic and is actually a poor environment for sperm survival. However, at certain times of a woman’s monthly cycle a woman produces a fluid (called cervical mucus) that is optimal for sperm survival. When sperm are in this special fluid, they can live from 3 to 5 days. If no cervical mucus is present in the woman’s vagina, sperm will die within minutes.

Men continuously produce sperm throughout their life span.  Women, on the other hand, have all of the eggs they will ever have at the time of birth. During a women’s monthly menstrual cycle, several eggs will begin to develop and one or more will mature in a tiny structure called the follicleOvulation occurs when an egg fully matures in the follicle and is ejected from the ovary. Ovulation occurs on only one day during a woman’s cycle.  The mature egg (or eggs) once released will live approximately one day. Therefore, in order for a woman to become pregnant and for a couple to conceive a baby, three factors need to be present; good sperm from the man, a good egg from the woman, and good cervical mucus for sperm survival.


There are three basic phases of a woman’s menstrual cycle: pre-ovulation (the time before ovulation); ovulation (the time the egg is released by the ovary); and post ovulation (the time after the egg dies).

The first phase of the woman’s menstrual cycle begins on the first day of her monthly bleeding (menses or period) and ends on the day of ovulation (the day the woman releases a mature egg). A woman has thousands of eggs, which are housed in two small sacks called ovaries. There is one ovary on each side of the woman’s body. The ovaries are attached to fallopian tubes, which are long and thin. These tubes are connected to the womb and help to guide the egg to reach the uterus. It is in the fallopian tubes where, if present, sperm will meet egg. In the first part of the menstrual cycle, one or more of the thousands of undeveloped eggs in a woman’s body begin to develop on a signal from a chemical messenger (hormone) from the brain called follicle stimulating hormone (FSH) (see Figure 2).

As the egg develops and matures in the follicle, it gives off another important chemical or hormone called estrogen (see Figure 2). Estrogen is important for fertility because it helps the inside of the uterus (womb) to develop. It also signals special cells in the opening of the uterus to produce cervical mucus.  This mucus is called cervical mucus. Cervical mucus is an important natural indicator of fertility.

The length of the pre-ovulation phase of the cycle can change from cycle to cycle in the same woman. For example, a woman could ovulate (release an egg) on day 14 in one cycle, and in the next cycle she could ovulate on day 10. Research has revealed that a variety of factors can effect this time of the cycle. Some of those factors include: weight loss, emotional stress (good or bad),illness, too much exercise, and even diet. Despite this variety, the time of fertility can be known by a woman. A woman’s body produces a number of biological signs (or markers) which can help her know when her fertility begins, when she ovulates, and when her fertility ends.

The second phase of the menstrual cycle is ovulation.  Ovulation happens when another body chemical (Luteinizing Hormone or LH) signals the ovary to release a mature egg (see Figure 2).  The releasing of the egg is called ovulation and is the optimal time of fertility. At this time some women experience breast tenderness and swelling, abdominal pain or cramping, and vulvar swelling.  Furthermore, at this time, the cervical opening (OS) widens, softens, and fills with sperm friendly (clear-slippery) mucus.

The next and final phase of the woman’s menstrual cycle begins the first day after ovulation and ends the day before the next period. This time in the woman’s menstrual cycle is called the luteal phase and is relatively stable in length (averaging about 10-16 days in all women). 


Fig 2: Female Menstrual Cycle and Hormones

Hormone Levels – for 28-day cycle:

After ovulation, the body increases the release of another female chemical, a hormone called progesterone (see Figure 3).  Progesterone has a number of important functions. It elevates the woman’s body temperature about 0.4 – 0.8 tenths of a degree Fahrenheit. This heating up of the woman’s body can be detected by taking daily waking temperatures. Progesterone also prepares the lining of the uterus for possible implantation of a new human being. Finally, progesterone stimulates cervical cells to produce thick mucus that closes off the opening of the cervix and thus serves as a barrier to sperm and bacteria.

If an egg is fertilized by a sperm (i.e., the man’s sperm cell unites with the woman’s egg cell) a new human life will be created. The new person will travel down the fallopian tube in a 6-9 day journey. On about day 10, the baby attaches to the wall of the mother’s uterus. The uterus will then house and feed the new baby for the next 9 months. If the woman’s egg is not fertilized by the man’s sperm, the levels of progesterone will decrease, and the lining of the uterus will shed. This shedding of the lining of the uterus is experienced by the woman as bleeding and is called menstruation.