Explainer: Understanding Sperm, Semen, and How a Baby’s Sex Is Decided

 What to Know About Semen

The Fertility and IVF (In Vitro Fertilization) Center defines semen as a fluid made up of sperm and secretions produced by various glands. It is thick, sticky, and whitish grey in color, released from the male reproductive tract during ejaculation. Its primary function is to carry sperm through the female reproductive system to enable fertilization.

According to the center, semen serves as the vehicle for sperm, the male reproductive cells. Sperm cannot survive or reach the egg through unhealthy semen, which directly reduces the chances of conception.

Semen production depends on several organs working in coordination: the testicles, which produce sperm cells through spermatogenesis; the epididymis, where sperm mature and are stored until ejaculation; the seminal vesicles, which produce most of the semen fluid, including fructose for energy; the prostate gland, which adds fluid that supports sperm motility and survival; and the urethra, which carries semen through the penis during ejaculation. Together, these organs ensure sperm are mixed with the right combination of fluids to support fertilization.

Semen quality is influenced by lifestyle, environmental, and health factors, including age, smoking, alcohol use, obesity, heat exposure, poor diet, and stress.

Varbah A. Sannoh, a Resident Doctor and Gynecologist (OB/GYN) at the John F. Kennedy Medical Center, told The Stage Media that semen is a broad term covering the combination of the sperm cell and other fluids. “When the sperm leaves the site of production, other prostatic glands add fluid to it to increase its mobility and act as a preservative, keeping it healthy,” Sannoh explained.

He said a semen analysis evaluates the combination of all fluids mixed with the sperm cell to assess quality, quantity, and motility. “When sperm enters the vagina without semen, the environment kills it. Those fluids are added to protect the sperm long enough for it to reach the egg for fertilization,” he added.

Verywell Health describes sperm as the reproductive cells produced by the testicles. They carry the male partner’s genetic material—chromosomes—and during sexual intercourse, travel through the vagina to the uterus and fallopian tubes, where fertilization can occur. The combined genetic material from both parents shapes the baby’s traits.

The testes produce between 100 and 200 million sperm cells daily through a process called spermatogenesis, which takes approximately 74 days to complete.

Each sperm cell has three parts: a head, containing enzymes that help it penetrate the egg membrane; a neck, housing mitochondria that power the cell; and a tail (flagellum), which propels it toward the egg through wave-like movements.

Every sperm carries 23 chromosomes representing the father’s genetic information. At fertilization, these combine with 23 chromosomes from the egg to form a zygote—a single cell with 46 chromosomes that becomes the foundation for a new life.

In humans, females inherit an X chromosome from each parent. Males always inherit their X chromosome from their mother and their Y chromosome from their father. This means all cells in human females carry two X chromosomes (XX), while all cells in males carry one X and one Y (XY).

The chromosome that the father contributes at the moment of conception determines the sex of the child. If the sperm carries an X chromosome, the child will be female; if it carries a Y chromosome, the child will be male.

Sperm production is continuous from puberty through old age. While female fertility ends with menopause, men can continue producing sperm for the rest of their lives—in rare cases, some have fathered children well into their 90s.

For most men, fertility begins to decline around age 40, as the accumulation of free radicals causes DNA damage that affects both sperm morphology (shape) and motility (movement).

The decline in sperm motility and morphology tends to be gradual, decreasing by approximately 1–2 percent per year from age 40.

Since the 1980s, the percentage of men under 30 who have conceived has fallen by more than 27 percent. Conversely, the conception rate among men aged 35 to 49 has increased by more than 60 percent.

Infertility is defined as the inability to conceive after at least one year of regular, unprotected intercourse. Around half of all infertility cases are linked to the male partner, with 90 percent of those cases involving sperm abnormalities.

A semen analysis—also called a sperm count—assesses the number, shape, and movement of sperm, as well as the acidity, volume, and appearance of semen. Because sperm counts fluctuate from day to day, testing typically involves two or three samples taken at least seven days apart.

A count below 15 million sperm per milliliter of semen, or fewer than 39 million per ejaculation, is considered low. This condition is medically termed oligozoospermia.

The duration of sperm survival plays a critical role in conception. Under the right conditions, sperm can remain viable in the female reproductive tract for up to five days, though survival rates drop sharply over time. Studies suggest that by day four, only around 5 percent of sperm remain viable.

Some factors that lower sperm count are modifiable, meaning they can be addressed through lifestyle changes, while others are not. A sperm analysis is always recommended to understand the full picture.

Sannoh told The Stage Media that in medicine, a person under 35 who has regular unprotected intercourse for one year without conceiving is considered potentially infertile. For those above 35, the threshold is six months. “As you age, your fertility window narrows. If you’re having regular unprotected intercourse for six months without a pregnancy and you’re over 35, that warrants investigation,” he said.

He said fertility investigations should begin with the man since the process is less complex and easier to resolve than investigations for women.

“The investigation looks at whether the man is producing enough sperm. Each ejaculation should produce between 2 and 5ml of semen. Anything below 2ml indicates a reduced sperm count in that ejaculation, which makes impregnating a woman difficult. The cells responsible for sperm production must also be functioning adequately,” Sannoh explained.

He added that sperm must be genetically intact. “Anything involving genetic abnormalities makes it impossible for a woman to become pregnant. Having enough sperm cells is not enough; they must also be genetically healthy.”

“We also examine the transport pathway. Sperm must travel from the male genital tract to the female genital tract, and that route must be clear. It is equally important to determine whether the man can ejaculate with enough force,” Sannoh said.

He further noted, "Everything can be functioning, but if you cannot ejaculate with sufficient force into the female genital tract for fertilization to occur, that is where impotence becomes a factor. Ideally, each ejaculation should contain between 15 and 20 million genetically healthy sperm cells.”

Sperm motility is also critical. Sperm must move upward toward the egg. If an analysis shows that the majority of sperm are moving backward, fertilization cannot occur.

“There are conditions that can cause a drop in sperm count. A man who has previously fathered children may later be unable to do so due to infections. One common culprit is chlamydia, a bacterial infection that affects the site of sperm production and reduces output,” Sannoh said.

Sannoh said few men seek medical treatment for fertility issues, largely due to the cultural belief that infertility is a woman’s problem. Often, women begin blaming themselves before any medical investigation is conducted.

“Male infertility can be treated depending on its cause. Infections like chlamydia acquired in childhood can be addressed with antibiotics,” he noted.

He also described cryptorchidism, a condition where the testicle fails to descend from the abdomen to the scrotum at birth, as one cause of infertility. “Normally, the testicle descends as a male ages. When this does not happen, pregnancy becomes impossible. However, the condition can be corrected through surgery to reposition the testicle.”

Sannoh explained the biological reason for external testicles: “The testes are located outside the body because testicular temperature must remain 1.5 to 2 degrees Celsius below normal body temperature. At higher internal temperatures, sperm cells are killed before ejaculation.”

He urged women not to blame themselves when conception does not occur. “Even if a man has children, do not assume the problem is yours. Those children may not be his. Statistically, 40 percent of fertility problems originate with women, 30 percent with men, 30 percent involve both partners, and 10 percent are of unknown cause,” he noted.

Sannoh said women tend to attract more blame due to the complexity of their reproductive systems and the role of hormonal fluctuations, but men carry significant responsibility as well.

Conclusion: Male infertility related to sperm may involve oligozoospermia (low sperm count), teratozoospermia (abnormal sperm shape), asthenozoospermia (poor sperm motility), or azoospermia (complete absence of sperm). Treatment of these conditions may require consultation with a urologist specializing in male infertility or a reproductive endocrinologist specializing in hormonal disorders affecting fertility.

Declaration: This article was developed through research conducted across multiple medical websites and an interview.