Storks and babies have a knack for showing up at the same time.

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Introduction to How Human Reproduction Works

In the Middle Ages, the summer solstice was a big event for Europeans. Weddings were planned for that day, and many communities held large parties with plenty of adult beverages. As a result, nine months after that day, quite a lot of babies would be born. Coincidentally, white storks returned from their migratory travels exactly nine months after the summer solstice as well, and it's believed that storks gained their reputation for bringing babies to mothers because of this scheduling sync-up [source: Adams].

Many adults may still turn to the story of the stork when they want to avoid an awkward conversation of how babies are made, but here at HowStuffWorks.com, we don't shy away from any of the tough questions. In this article, we'll explore the biology of sex -- otherwise known as human sexual reproduction. We'll examine the body's sexual organs, the biological cycles of sex and the process of fertilization. If you need a refresher on the birds and the bees, this is the article for you.

There are many reasons why people have sexual intercourse -- it improves intimacy between a couple by releasing hormones that help them bond, and studies have shown that sexual activity relieves stress, boosts immunity, reduces pain and burns calories [source: Doheny]. Those are benefits that anyone can reap from sex, no matter their gender or their sexuality. But for the subject at hand -- making a baby -- a man and a woman and their unique genetic information is required. On the next few pages, we'll discuss the reproductive systems of men and women.

She may not know if she's painting the nursery pink or blue yet, but the decision has already been made.

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Development of Sex Organs

Many expectant parents look forward to the doctor's appointment that comes 20 weeks into the pregnancy -- that's when an ultrasound can usually reveal the sex of their baby. The sex of that baby is determined at the exact moment when egg and sperm meet, but it takes a while for the external organs to match the internal chromosomes.

As an embryo develops, it acquires both Wolffian and Mullerian ducts. Wolffian ducts develop into male sex organs, and Mullerian ducts develop into female sex organs. Which sex organs develop depends on the presence of a Y chromosome and the male hormone testosterone and anti-Mullerian hormone (AMH). At eight weeks, the internal genitalia will begin to form. If the embryo has both an X and a Y chromosome and produces the two hormones, then the testosterone will stimulate the Wolffian duct to develop male sex organs, including the vas deferens and the seminal vesicles. If there's no Y chromosome, but two X chromosomes instead, then the embryo is female. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs such as the uterus, fallopian tubes and part of the vagina. Rarely, the embryo will have an X and a Y chromosome, but will fail to produce testosterone or AMH; such an embryo is termed intersex, as it has both male and female sex organs.

The external genitals continue to develop after the internal ones have formed. Testosterone produces a penis and a scrotum for the male embryo, while the lack of testosterone will lead to a clitoris, urethra, the rest of the vagina and the labia for the female. These organs will continue to develop during pregnancy, and then they will undergo another period of development at puberty. On the next page, we'll take a closer look at the functions these organs perform.

Male sex organs

Male Sex Organs

­From the outside, the male has two visible sex organs, the testes (or testicles) and penis. The testes are the primary male sexual organs in that they make sperm and produce testosterone. The sperm is the male sex cell, or gamete, and each testicle produces more than 4 million new sperm per hour [source: Angier]. Testosterone is the hormone that causes male secondary sex characteristics such as facial and pubic hair, thickened vocal cords and developed muscles.

The testes are housed outside of the main part of the male's body, in a sac called the scrotum. This location is important because in order for the sperm to develop properly, they must be kept at a temperature about two degrees cooler than normal body temperature. It takes sperm about 4 to 6 weeks to mature, which they do as they travel from each testis to a coiled tube on the outer surface of each testis called the epididymis. Sperm, which are often compared to tadpoles in appearance, use their tails to travel, while the head contains the genetic material.

The penis is made of soft, spongy tissue that can expand or contract. When a man becomes sexually aroused, the penis becomes engorged with blood, causing the spongy tissues to stiffen and the penis to become erect. This erect state makes it easy to place the penis inside a woman's vagina during sexual intercourse. During sex, the male reproductive system is still at work, equipping the sperm with the assistance they will need to fertilize an egg. Find out more about this process on the next page.

Delivering Sperm Cells

As we mentioned on the last page, sperm are made in the testes. During sexual intercourse, smooth muscles contract and propel mature sperm from the end portions of the epididymis through a long tube called the vas deferens, which is inside the body, just beneath the bladder. From there, the sperm get mixed with nutrient-rich fluids from the seminal vesicles and a milky secretion from the prostate gland. This combination of sperm and fluids is called semen. The semen does three things:

  • Provides a watery environment in which the sperm cells can swim while outside the body
  • Provides nutrients for the sperm cells (fructose, amino acids and vitamin C)
  • Protects the sperm cells by neutralizing acids in the female's sexual tract

Once the semen is made, it passes through the urethra, which is within the penis, exiting the body through a small slit at the tip of the penis in a process known as ejaculation. The average ejaculate contains about one teaspoon of seminal fluid, and only one percent of the mixture is sperm [source: Angier].

One last male reproductive organ is a tiny, pea-sized set of glands inside the body at the base of the penis, called the bulbourethral or Cowper's glands. During sexual excitation, and just prior to the ejection of sperm, the Cowper's glands secrete a tiny amount of fluid that neutralizes any traces of acidic urine that may be leftover in the urethra. It is also believed that these secretions are designed to lubricate the penis and female tract during sexual intercourse.

Female Sex Organs

Female Sex Organs

The two ovaries are the major female sex organs, the counterpart of the male testes. The ovaries make the eggs, or oocytes, which are the female gametes, and produce estrogen, the female sex hormone. Estrogen causes female secondary sexual characteristics such as pubic hair, breast development, widening of the pelvis and deposition of body fat in hips and thighs. The ovaries are located in the abdomen.

Eggs develop inside the ovary and are released upon ovulation into the Fallopian tube, which is lined with fingerlike projections that push the egg through the tube. During the egg's travels through the Fallopian tube, fertilization can take place if sperm is present, and then the fertilized egg will continue to a muscular chamber called the uterus.

The uterus, or womb, is where a fetus develops. It's made of smooth muscle and is normally about the size and shape of a small pear turned upside down. During pregnancy, it can stretch to about the size of a basketball to hold the developing baby. The base of the uterus (the neck of the pear) is a muscular wall called the cervix. In the cervix is a tiny opening, about the size of a pinhead, called the external os. The external os is filled with a thick plug of protein (mucus) that serves as a barrier to the entrance of the uterus. The cervix leads into a smooth-muscle-walled tube called the vagina, or birth canal.

The vagina connects the uterus to the outside of the body, and its opening is covered by sets of folded skin called labia. The vagina receives the male's penis during sexual intercourse and delivers the baby during childbirth. The vagina is normally narrow, but can stretch during intercourse and childbirth.

Finally, two sets of glands, the greater vestibular gland (Bartholin's gland) and the lesser vestibular gland, are located on either side of the vagina and empty into the labial folds of skin. The secretions from these glands lubricate the labial folds during sexual excitation and intercourse.

Other Sexual Organs

Although not located in the reproductive systems, two other organs are important for sexual function in both males and females:

  • The hypothalamus in the brain -- The hypothalamus has nerve cells that secrete a hormone called gonatotropin releasing hormone (GnRH) into the blood vessels leading to the anterior pituitary gland.
  • The anterior pituitary gland just beneath the brain -- GnRH causes the anterior pituitary cells to release two hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH), into the general blood circulation. LH and FSH act on the testes/ovaries to stimulate the making and maturation of the sex cells and the production of sex hormones (testosterone, estrogen and progesterone).

The nerve cells time-release small, low-level spurts of GnRH every 90 minutes, which causes the anterior pituitary to secrete small pulses of LH and FSH. The sex hormones from the testes/ovaries give feedback to the hypothalamus and anterior pituitary to regulate the secretion of GnRH, LH and FSH -- this interplay is called the negative feedback control system. The chemical interplay between the hypothalamus, anterior pituitary gland and the testes/ovaries is important for sexual development, maintaining sexual function and sexual reproduction. An error in this chemical interplay can be a cause of infertility.

DNA: The Molecule of Life

Photo courtesy U.S. DOE, Human Genome Project

Production of Sex Cells

From the time of puberty on, men make sex cells (in the form of sperm cells) continuously. In contrast, by the time a woman is born, she has made all of the eggs that she will ever have. As she reaches puberty, the eggs begin to develop and get released, and this process continues until menopause. In both males and females, the production of sex cells involves meiosis, a type of cell division whereby our two sets of genetic instructions are reduced to one set for the sex cell.

Each cell in the human body is made up of 46 chromosomes -- 23 from the mother's egg and 23 from the father's sperm. These human chromosomes are X-shaped, except for the male Y chromosome. When the body produces sex cells, like sperm or eggs, there are 46 chromosomes present, but since the sex cell will merge with its counterpart, only 23 are needed. The body must then reduce the number of chromosomes by half to go into the sex cells.

To do this, it randomly sorts chromosomes from both sets in one cell division and then reduces them by half in another. Therefore, each sperm or egg that the body produces is unique -- it contains a different mix of the mother's and father's genes. This is why two brothers in the same family can look and act totally different from one another even though they come from the same parents -- it all depends on which genes (chromosomes) were randomly chosen when producing the sex cells of the mother and father.

Timing is everything when it comes to female fertility.

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Reproductive Cycles

Remember that, biologically, the main goal of sexual reproduction is to have the sperm combine with the egg to make a baby. With respect to the man, age is not critical. Most men are capable of producing sperm that can fertilize an egg at any time from the onset of puberty until they die; there have been cases in which men in their 70s and 80s have conceived children with younger women. Women, on the other hand, have something of a limited window for fertility. They typically can release fertilizable eggs from the time of puberty until their late 40s or early 50s. After that time, their ovaries stop releasing eggs and they undergo various biochemical and physiological changes, the sum of which is called menopause.

Although women are capable of conceiving a child from puberty to menopause, the timing of sexual intercourse is critical to the success of sexual reproduction. Women have an ovulatory -- or menstrual -- cycle, made up of complex physiological and biochemical changes that are linked to the timing of reproduction.

In the first phase of the menstrual cycle, a follicle (the cell complex that surrounds and nurtures the egg) grows in the ovary, while the lining of the uterus builds up to receive a fertilized egg. At mid-cycle, when the egg is ready, the ovary releases the egg. This stage is called ovulation, and is the optimum time for fertilization to take place. The egg enters one of the Fallopian tubes on its way to the uterus. In the last phase of the menstrual cycle, one of two things happens: If the egg is fertilized, it continues on to the uterus, attaches to the lining and pregnancy begins, but if the egg is not fertilized, the uterus sheds its lining and the now-dead egg, menses begins and another menstrual cycle ensues.

Forget what you've read about certain sexual positions ensuring a boy or a girl -- just have fun.

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Intercourse

So far we've talked about sex cells and sex organs, and you might be wondering when we'd get around to the act of sex itself. Well, here we go -- the moment when the male and female sex organs and sex cells get together. While there are many ways in which a man and a woman can engage in sexual behavior together, vaginal intercourse, in which a man's penis is inserted into a woman's vagina, is the method by which sperm meets egg.

As we've discussed, the male and the female reproductive system are designed to work together for this purpose -- the penis becomes erect to facilitate entrance to the vagina, for example, and both men and women excrete fluids that make that passage easier. These steps are part of the sexual response cycle. Both men and women experience the various phases of the cycle, from excitement to plateau to orgasm to resolution.

During the first two phases, both men and women's bodies are undergoing physical changes which include increased blood flow to the genitals as well as an increase in pulse, blood pressure and body temperature. As intercourse continues, the man reaches a point at which muscle contractions in the epididymis, prostate and seminal vesicles propel semen from the penis into the woman's vagina. The man's orgasm almost always happens at the same time as this ejaculation, and after that, the man begins to enter the resolution phase. Men and women experience the sexual response cycle at different speeds, so a woman may not experience an orgasm at the same time as a man, which has led to speculation that the female orgasm is a "happy accident," as it's not necessary for fertilization. Some researchers, however, believe that if a woman orgasms when a man does, the muscle contractions in her body will cause the cervix to dip and contract, making it easier for the sperm in the semen to begin their journey.

And what a journey it is -- on the next page, we'll consider the incredible odds against the sperm fertilizing an egg.

Sperm on the hunt for an egg

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Fertilization

Fertilization is the union of sperm and egg and the fusion of the chromosomes necessary for the formation of an embryo. Though some people who find themselves pregnant may think that the process is all too easy, it's actually a remarkable feat for a sperm to fertilize the egg, as the odds are stacked against it.

The average ejaculation contains about 150 million sperm, but the majority of them will prove unfit for travel; researchers have found that about 85 percent of sperm have an abnormality that will prevent it from reaching and fertilizing the egg [source: Angier]. The vagina and cervix do their part to encourage the sperm, emitting chemical signals that show the path for the seminal swimmers.

It's a race against time, however: The journey to fertilization must be completed within 12 to 48 hours, before the sperm die. And timing is crucial on the other end, as well. For fertilization to take place, sperm must be present either a couple of days prior to or on the day of ovulation.

Sperm must first cross the barrier of the cervix, which will be thin and watery if the woman has just ovulated. Once the sperm have traversed the cervical mucus, they travel up the moist lining of the uterus into the Fallopian tubes. Since only one of the Fallopian tubes contains an egg, many sperm travel in the wrong direction. Fewer than 1,000 sperm out of the millions in the semen reach the Fallopian tubes, and only a few dozen actually reach the membrane of the egg.

The Fertilization Process

The lucky few sperm who reached the egg in the Fallopian tube surround it and begin competing for entrance. The head of each sperm, the acrosome, releases enzymes that begin to break down the outer, jelly-like layer of the egg's membrane, trying to penetrate the egg. Once a single sperm has penetrated, the cell membrane of the egg changes its electrical characteristics. This electrical signal causes small sacs just beneath the membrane (cortical granules) to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg in a process called cortical reaction. The cortical reaction ensures that only one sperm fertilizes the egg. The other sperm die within 48 hours.

The fertilized egg is now called a zygote. The depolarization caused by sperm penetration results in one last round of division in the egg's nucleus, forming a pronucleus containing only one set of genetic information. The pronucleus from the egg merges with the nucleus from the sperm. Once the two pronuclei merge, cell division begins immediately.

The dividing zygote gets pushed along the Fallopian tube. Approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst. When the blastocyst reaches the uterine lining, it floats for about two days and finally implants itself in the uterine wall around six days after fertilization. This signals the beginning of pregnancy. The implanted blastocyst continues developing in the uterus for nine months. As the baby grows, the uterus stretches until it's about the size of a basketball.

Double trouble?

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Twins

Sometimes, two dominant follicles develop eggs and ovulate. If both are fertilized and subsequently implanted in the uterus, two embryos develop: twins. Because they developed from separate eggs that were fertilized by different sperm, they are called fraternal twins. Fraternal twins do not share any more genetic information than siblings born separately.

In addition, the two daughter cells that remain after a fertilized egg undergoes its first division may separate and divide independently of each other. When this happens, they remain loosely connected while in the Fallopian tube, and the two blastocysts implant together in the uterine wall. They develop into two separate embryos. Because these embryos came from the same fertilized egg, they share identical genetic material and are called identical twins.

The chance of having twins skyrocketed between 1980 and 2004, when the rate of twin births in the U.S. increased by 70 percent [source: March of Dimes]. Researchers attribute this jump to the increased use of fertility drugs and treatments, which can increase the chances of multiple births. However, in recent years, the rates of twin births have stabilized, likely due to increased knowledge and more precise application of reproductive technology. According to the most recent data from the Centers for Disease Control and Prevention, in 2008, there were 32.6 twin births for every 1,000 births, for a total of 138,660 sets of twins born [source: CDC].

Contraception and STDs

While this article detailed the steps of human reproduction, there are ways to stop such a thing from happening. There are several methods of contraception:

  • Abstinence involves not engaging in sexual activity.
  • Birth control pills prevent ovulation.
  • Condoms, intrauterine devices, cervical caps and diaphragms place a barrier between sperm and egg.
  • Spermicides kill sperm.
  • Surgical procedures such as tubal ligations for women or vasectomies for men will cease ovulation and sperm production.
  • Calendar-based methods involve determining periods of maximum fertility and avoiding sexual contact during those times.

These methods have varying degrees of effectiveness, and not all of them may be suited for certain people. Talk with a medical professional about the method that's right for you and your situation.

And even if you're not planning on reproducing right this minute, you may want to someday. That's why it's important for men and women to keep their reproductive health in tip-top shape. Sexually transmitted diseases and infections can cause fertility trouble down the road. Common sexual infections include gonorrhea, syphilis, chlamydia, genital herpes, HIV/AIDS and trichomoniasis. To prevent sexual infections, both men and women should be regularly tested and engage in safe sexual activity.

Other issues can affect fertility as well. For women, age is a factor, as are hormonal imbalances, stress, poor diet, autoimmune disorders and tobacco and alcohol use. If a woman is either underweight or overweight, she may have trouble conceiving. For men, poor diet, obesity, stress, tobacco, alcohol, drugs, certain medications and very hot water can all affect sperm quality.

For more information on human reproduction and related topics, follow the links on the next page.

Lots More Information

Sources

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  • Angier, Natalie. "Sleek, Fast and Focused: The Cells That Make Dad Dad." The New York Times. June 12, 2007. (July 26, 2011) http://www.nytimes.com/2007/06/12/science/12angi.html
  • Centers for Disease Control and Prevention. "Multiple Births." Oct. 5, 2010. (July 26, 2011) http://www.cdc.gov/nchs/fastats/multiple.htm
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  • Encyclopaedia Britannica. "Human sexual behavior." 2011. (July 26, 2011) http://www.britannica.com/EBchecked/topic/537102/human-sexual-behaviour
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