The process of reproduction in humans usually relies on sexual intercourse between a male and a female, although there are exceptions to this. Unlike many animals, humans mate throughout the year. Humans have sexual intercourse when sexual reproduction is not possible for reasons such as the use of birth control or female menopause. Practices and behaviors surrounding human reproduction vary widely across cultures, but in every case, it involves sperm, an ovum (egg), a uterus and a baby.
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During meiosis, diploid cells divide into sperm in males and ova in females. During sexual intercourse, the male ejaculates semen, containing hundreds of millions of sperm into the vagina. If the female is ovulating, a sperm may encounter an ovum. When a sperm cell penetrates the ovum's barrier, its 23 chromosomes fuse with the ovum's 23, forming the zygote.
The zygote divides and multiplies many times. The growing embryo travels to the uterus, where it remains, and about 40 weeks after fertilization, a baby is born.
The process of reproduction in humans begins with meiosis. In human meiosis, diploid cells with the usual 46 chromosomes divide into four haploid daughter cells, each containing 23 chromosomes. Each of these daughter cells is called a gamete. In males, this meiotic process is called spermatogenesis, and the daughter cells are sperm. In females, the process is called oogenesis, and the daughter cells are called ova. Males begin spermatogenesis at puberty and continue throughout life. Healthy young adult males produce hundreds of millions of sperm each day. This number begins to decline by their mid-20s.
Unlike males, females begin to produce gametes before they even are even born. By the fifth month in the womb, female fetuses have begun oogenesis, but the process pauses after a phase called prophase I, suspending the ova in the primary oocyte stage until puberty. 99.9 percent of a female’s ova remain in the primary oocyte stage until they are eventually absorbed by the body. Millions are absorbed by the time a fetus is born, and by puberty, only 400,000 remain. For every ovulation, about 2,000 more ova are absorbed.
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The four stages of the human sexual response cycle occur during partnered sex with people of any gender, as well as other sexual stimulation activities. The first stage is excitement, the beginning of arousal, in which blood flow increases and causes engorgement in the genitals and nipples, accompanied by an increase in heart rate, breathing rate, muscle tone and blood pressure. Next is the plateau stage, which is brief, and which involves an increase in arousal.
The third phase is orgasm, which involves waves of muscle spasms and pleasure that last several seconds. During this phase, the uterus has several contractions, and the penis has contractions at its base, causing semen, the fluid containing sperm, to ejaculate into the vagina. The last stage is resolution, during which the body relaxes to its original state.
Fertilization and Cleavage
The sperm take several minutes to travel through the vagina, cervix and uterus and reach the fallopian tubes. Out of hundreds of millions of sperm, one or two hundred make it that far. If the female is ovulating, the sperm can survive for up to 48 hours as the ovum travels down a fallopian tube from an ovary to meet the sperm. If the ovum is already in the fallopian tube, it can only survive 24 hours before the sperm reach it.
The ovum is encased in a protective coating called the zona pellucida. The sperm that reach the zona pellucida bind to it and then try to penetrate it. Eventually, one sperm succeeds, which causes chemical changes. This destroys the zona pellucida’s sperm receptors so that no other sperm bind to it, and the zona pellucida hardens, blocking any remaining sperm attempting to cross the barrier. The sperm that made it through fuses with the ovum. The result is a zygote – a one-celled diploid embryo.
Gestation and Birth
The zygote undergoes a process called cleavage, in which it replicates itself by mitosis, and then continues to replicate, forming a multicelled blastocyst. The growing embryo travels from the fallopian tube to the uterus and attaches to the uterine lining, the endometrium, between days five and seven. Over the next few days, the embryo moves away from the endometrium and extends cells into it that become the umbilical cord and the placenta. The embryo receives nutrients and expels wastes via the umbilical cord.
By the eighth week, the embryo has become a fetus, with four limb buds and most of its major organ systems formed and external genitalia beginning to develop. During the second trimester, the fetus grows and develops its skeleton. Its movements become detectable by the parent. During the third trimester, the fetus continues to grow, and its respiratory and circulatory systems prepare for it to breathe air.
The process of birth typically happens after 40 weeks. It begins with the rupturing of the amniotic sac, which contained and protected the fetus, and the fluid inside spills out, which is known as “water breaking.” Hormones, especially oxytocin and prostaglandins, dilate the cervix and cause increasing uterine contractions to guide the fetus out through the birth canal. Over the course of minutes, hours or even days, the fetus is pushed out of the womb by uterine contractions followed by the placenta.
Sexual Reproduction Model
Some reproduction does not require intercourse but is the result of artificial insemination when a couple has fertility problems or a single prospective parent or a same-sex couple choose a sperm donor. Also, while male and female are simple terms for the biological processes of reproduction in humans, this language excludes the sexuality of transgender and intersex people. For example, a cisgender man (a man whose gender matches his birth sex) and a transgender man (a man who was assigned female at birth) who has not undergone sex reassignment surgery can have sexual intercourse with each other, and the transgender man can become pregnant.