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Sperm are manufactured in the testes and ejaculated out through the vas deferens and urethra into the vagina. Then they begin their swim through the cervix and uterus and up a fallopian tube to meet the egg, which has already been released from the ovary. When the sperm and egg unite in the fallopian tube, conception occurs. The single fertilized egg cell then begins dividing as it travels down the tube, and finally it implants in the uterus. Various techniques for improving the chances of conception are available.

The placenta, which is important in transmitting substances between the woman and the fetus, develops early in pregnancy. The most remarkable development of the fetus occurs during the first trimester (first three months), when most of the major organ systems are formed and human features develop.

For the woman, early signs of pregnancy include amenorrhea, tenderness of the breasts, and nausea. The most common pregnancy tests are designed to detect hCG in the urine or blood. Physical changes during the first trimester are mainly the result of the increasing levels of estrogen and progesterone produced by the placenta. Despite cultural myths about the radiant contentment of the pregnant woman, some women do have negative feelings during the first trimester. During the second trimester the woman generally feels better, both physically and psychologically.

Despite people’s concerns, sexual intercourse is generally quite safe during pregnancy. Nutrition is exceptionally important during pregnancy because the woman’s body has to supply the materials to create another human being. Pregnant women must also be very careful about ingesting drugs because some can penetrate the placental barrier and enter the fetus, possibly causing damage.

Labor is typically divided into three stages. During the first stage, the cervix undergoes effacement (thinning) and dilation. During the second stage, the baby moves out through the vagina. The placenta is delivered during the third stage. Cesarean section is a surgical method of delivering a baby.

The Lamaze method of “prepared” childbirth has become very popular; it emphasizes the use of relaxation and controlled breathing to control contractions and minimize the woman’s discomfort. Anesthetics may not be necessary, which seems desirable, since they are potentially dangerous.

During the postpartum period, hormone levels are very low. Postpartum depression may arise from a combination of this hormonal state and the many environmental stresses on the woman at this time.

Two hormones are involved in lactation: prolactin and oxytocin. Breast-feeding has a number of psychological as well as health advantages.

Problems of pregnancy include ectopic (misplaced) pregnancy, pseudocyesis (false pregnancy), preeclampsia and eclampsia, illness (such as German measles), a defective conceptus, Rh incompatibility, spontaneous abortion, and preterm birth.

The most common cause of infertility in men and women is sexually transmitted infections.

Assisted reproductive technologies include artificial insemination, frozen sperm banks, embryo transplants, in vitro fertilization (test-tube babies), and GIFT (gamete intrafallopian transfer), all of which are now a reality. These procedures are expensive and have low success rates. In addition, the practice of transferring multiple embryos often results in multiple births, which are riskier for both mother and infants.

William Masters and Virginia Johnson conducted an important program of research on the physiology of human sexual response. They found that two basic physiological processes occur during arousal and orgasm: vasocongestion and myotonia. The sexual response cycle occurs in three stages: excitement, orgasm, and resolution.

Their research indicates that there is no physiological distinction between clitoral and vaginal orgasms in women, which refutes an early idea of Freud’s. They also provided convincing evidence of the existence of multiple orgasm in women.

Criticisms of Masters and Johnson’s model are that (1) they ignored cognitive factors and (2) their selection of research participants may have led to a self-fulfilling prophecy in their results.

Two cognitive–physiological models are Kaplan’s three-component model, focusing on desire, vasocongestion, and muscular contraction, and Walen and Roth’s model, which emphasizes cognitive aspects of sexual response, namely perception and evaluation.

The nervous system and sex hormones are important in sexual response. The nervous system functions in sexual response by a combination, of spinal reflexes (best documented for erection and ejaculation) and brain influences (particularly of the limbic system). There is evidence that some women ejaculate. Hormones are important to sexual behavior, both in their influences on prenatal development (organizing effects) and in their stimulating influence on adult sexual behavior (activating effects). Testosterone seems to be crucial for maintaining sexual desire in both men and women.

Pheromones are biochemicals secreted outside the body that play an important role in sexual communication and attraction. Much of the evidence is based on research with animals, but evidence in humans is accumulating rapidly.

Sexual pleasure is produced by stimulation of various areas of the body called erogenous zones.

Sexual self-stimulation, or autoeroticism, includes masturbation and sexual fantasies. Many people have sexual fantasies while masturbating. Common themes of these fantasies are kissing and touching sensuously, oral sex, and seduction. Similar sexual fantasies are also common during intercourse.

An important technique in two-person sex is hand stimulation of the partner’s genitals. A good guide to technique is to find out how the partner masturbates. Touching other areas of the body and kissing are also important. The other senses—sight, smell, and hearing—can also be used in creating sexual arousal.

While there are infinite varieties in the positions for intercourse, there are four basic positions: man on top (the missionary position), woman on top, rear entry, and side to side.

The two kinds of mouth–genital stimulation are cunnilingus (mouth stimulation of the female genitals) and fellatio (mouth stimulation of the male genitals). Both are engaged in frequently and are considered pleasurable by many people. Lesbians and gays use techniques similar to those of straights (e.g., hand–genital stimulation and oral– genital sex). Gays and lesbians, though, seem less goal oriented, take more time, and communicate more than heterosexuals do.

Anal intercourse involves inserting the penis into the rectum. This activity and the insertion of other objects in the anus must be done carefully to avoid injury or transmission of STIs.

An aphrodisiac is a substance that arouses sexual desire. There is no known reliable aphrodisiac, and some of the substances that are popularly thought to act as aphrodisiacs can be dangerous to a person’s health.

We have a tendency in our culture, perhaps a legacy of the Protestant ethic, to view sex as work and to turn sex into an achievement situation, as witnessed by expressions such as “achieving orgasm.” Such attitudes make sex less pleasurable and may set the stage for sexual failures or sexual disorders.

A capacity for sexual response is present from infancy. According to contemporary data, about 40 percent of U.S. children have masturbated by the time they reach puberty. Recent studies indicate that children begin masturbating at somewhat earlier ages now than a generation or so ago. Children also engage in some heterosexual play, as well as some same-gender activity.

During adolescence there is an increase in sexual activity. According to one theory, this activity is influenced by the interaction of biological factors (increasing testosterone level) and social and psychological factors (for example, sexually permissive attitudes). By age 15, nearly all boys have masturbated. Girls tend to begin masturbating somewhat later than boys, and fewer of them do masturbate. Attitudes toward masturbation are considerably more permissive now than they were a century ago. About 10 percent of adolescents have same-gender experiences to orgasm.

Today the majority of males and of females alike have premarital sex. This is a considerable increase over the incidence reported in the Kinsey studies, done 60 years ago. Adolescents today are considerably more likely to use a variety of sexual techniques, including mouth–genital sex. There is variation in the incidence of premarital intercourse among various racial and ethnic groups in the United States, and even greater variability from one country to another.

The predominant sexual standard today is one of “permissiveness with affection”; that is, sex is seen as acceptable outside marriage, provided there is an emotional commitment between the partners. Following Erik Erikson’s theory, experiences with sexuality can serve important functions in a person’s psychological development. They may be important, for example, in the process of becoming independent of parents and in establishing a viable moral system.