99 Fun Facts About Reproductive System

Which temporary organ attaches to the wall of the uterus during pregnancy to supply the baby with oxygen and food?

The placenta. It also carries the baby's waste away, and because it is delivered shortly after the baby, it is commonly known as the afterbirth.

What is the inner lining of the uterus, the layer shed during a period, called?

Endometrium. It thickens each cycle ready to receive an embryo, and if none implants only its upper layer is shed, leaving a deeper layer behind to rebuild it.

Shortly after an egg is released, sperm usually meet and fertilise it inside which structure?

Fallopian tube. The embryo normally drifts on to the uterus, and when it implants in the tube instead the result is an ectopic pregnancy, because the narrow tube cannot hold a growing embryo.

What is the narrow neck of the uterus, where it joins the vagina, called?

Cervix. A plug of mucus keeps it sealed throughout pregnancy, then in labour it thins and opens to about 10 centimetres so the baby's head can pass through.

What is the first cell of a new human, formed when a sperm fertilises an egg, called?

Zygote. Only one sperm gets in: the moment it fuses with the egg, the egg's outer coat changes to lock out all the others, and the new cell starts to divide.

What is the release of an egg cell from the ovary called?

Ovulation. A surge of hormone makes the ripe egg sac burst open, and some women feel it as a twinge low on one side of the belly, the side of the ovary that released the egg.

About how many days does a typical menstrual cycle last?

28 days is the average, and the word 'menstrual' fittingly comes from the Latin for month. Anything from 21 to 35 days still counts as a normal cycle.

A girl's first period is known as what?

Menarche. It is not the opening act of puberty: it typically arrives around age 12, about two years after the breasts begin to develop.

Which hormone, rising through the first half of the cycle, makes the lining of the uterus thicken?

Estrogen. It rebuilds the lining after each period, and its peak just before the egg is released also thins the mucus that sperm must swim through.

Menstruation is triggered by falling levels of which hormone?

Progesterone. The emptied egg sac turns into a temporary gland that makes it to hold the lining in place; with no pregnancy the gland breaks down, the level drops and the lining is shed.

The hypothalamus in the brain sets the reproductive system in motion by releasing which hormone?

Gonadotropin-releasing hormone, or GnRH. It runs low in childhood, and its rise at puberty is what switches the system on, prompting the pituitary to signal the ovaries or testes.

What does FSH, the hormone that ripens the egg-holding sacs in the ovaries each cycle, stand for?

Follicle-stimulating hormone. Follicles are the small fluid-filled sacs that hold the eggs, yet men make this hormone too: in the testes it keeps sperm production going.

A surge of which pituitary hormone triggers the release of a mature egg?

Luteinizing hormone. Home fertility kits work by spotting this surge in urine a day or two before the egg appears, and in men the same hormone tells the testes to make testosterone.

How many chromosomes does a human egg or sperm cell carry, half the number found in other body cells?

23. Almost every other cell in the body holds 46, so when an egg and a sperm fuse the two half-sets add up to a full set again, half of it from each parent.

Which type of cell division halves the chromosome number to make eggs and sperm?

Meiosis. The cell copies its chromosomes once but then divides twice, which is why each egg or sperm ends up with only half the usual number.

What is the general term for the body's production of eggs and sperm?

Gametogenesis. The sexes run it at very different rates: a man turns out well over a hundred million sperm every day, while a woman typically releases a single egg a month.

When are all of a woman's eggs formed?

Before birth. Every egg she will ever release is already in her ovaries when she is born, paused part-way through its development, and the stock only shrinks from then on.

An egg or sperm carries one set of chromosomes instead of the usual two, making it what kind of cell?

Haploid. Ordinary body cells are diploid, with one set from each parent, and if eggs and sperm were too, the chromosome count would double with every generation.

Newly made sperm cannot yet swim, so in which tightly coiled tube behind each testis do they mature and gain that ability?

Epididymis. Uncoiled, this single tube would stretch about six metres, and sperm learn to swim on the slow trip through it; their tails are built earlier, inside the testis.

Sperm are produced in which fine coiled tubes inside the testes?

Seminiferous tubules. These tightly coiled tubes make up most of each testis, and from puberty onwards the cells lining them turn out sperm continuously for the rest of a man's life.

Which cells, tucked between the sperm-making tubes of the testes, produce testosterone?

Leydig cells. They get to work remarkably early: in a male embryo they are already releasing testosterone by about the seventh week of development, long before puberty.

Which 'nurse' cells inside the sperm-making tubes of the testes feed and shield developing sperm?

Sertoli cells. Sealed tightly to one another, they also wall developing sperm off from the bloodstream, because the immune system would otherwise treat those sperm as foreign.

Why do a man's testes hang outside his body?

To stay cooler. Sperm form best a few degrees below body heat, so muscles in the scrotum pull the testes up close when it is cold and let them hang lower when it is warm.

Which tube, also called the sperm duct, carries sperm toward the ejaculatory duct?

Vas deferens. A vasectomy works by cutting and sealing this tube on each side, so a man still ejaculates as before but the fluid no longer carries any sperm.

Which pair of structures behind the bladder supplies most of the fluid that sperm travel in?

Seminal vesicles. Their thick fluid is rich in the sugar fructose, which sperm burn as fuel for swimming, and it makes up more than half of what a man ejaculates.

The bulbourethral glands are also known by what other name?

They add fluid to semen during ejaculation.

A sperm carrying which chromosome results in a male child?

Y. Every egg carries an X, so the father's sperm decides a baby's sex; in birds it runs the other way, with chromosomes called Z and W, and the mother's egg decides.

Which more potent hormone, which the body makes from testosterone, is linked to male-pattern baldness?

DHT, short for dihydrotestosterone. Before birth this same hormone is essential for forming a boy's external genitals, so its link to thinning hair only shows up decades later.

Which paired duct in the early embryo goes on to form the uterus and the upper part of the vagina?

Müllerian duct. Every early embryo has this pair and the Wolffian pair side by side; in a boy, a hormone from the newly formed testes makes the Müllerian ducts wither away.

Home pregnancy tests work by detecting which hormone in a woman's urine?

hCG, often called the pregnancy hormone. The embryo's outer cells start releasing it once they burrow into the wall of the uterus, which is why a test only becomes reliable around the first missed period.

Which female organ grows from the same embryonic tissue as the penis?

Clitoris. Early on the two sexes look identical, and testosterone decides what happens next: with it this shared tissue grows into a penis, and without it the same cells form a clitoris.

What happens early in pregnancy to produce identical twins?

One embryo splits in two. Both halves carry the same DNA, so identical twins are almost always the same sex, whereas fraternal twins grow from two separate eggs and are no closer than ordinary siblings.

Where in a baby boy's body do the testes first form?

Abdomen. The testes start on its back wall and travel down through the groin into the scrotum in the final months of pregnancy; one that stops short is called an undescended testis.

In most men, which testis hangs slightly lower in the scrotum?

The left. The two sides differ inside too: the left testis drains into the kidney's vein instead of straight into the body's main vein, thought to be why swollen veins in the scrotum mostly turn up on the left.

What is the tough, white fibrous capsule that directly encloses each testis called?

Tunica albuginea. The name comes from the Latin albus, white, the root of albino and of albumen, the white of an egg, and the ovaries are wrapped in a capsule of the same name.

From start to finish, producing a sperm cell takes roughly how long?

3 months, near enough: about ten weeks of building plus the trip out through the ducts. The batches overlap so heavily, though, that more than a thousand sperm are finished every second.

What is the tail of a sperm cell called?

Flagellum. It is driven by mitochondria packed tightly into the sperm's midpiece, just behind the head, which supply the energy for the long swim towards the egg.

The word 'sperm' comes from a Greek word meaning what?

Seed. The Greek sperma literally meant 'that which is sown', and botany still uses it that way, which is why plant words such as gymnosperm and endosperm carry it too.

How much mitochondrial DNA does a human sperm typically pass on to offspring?

Little or none. A baby's mitochondria come from the egg, so as a rule this DNA passes down the mother's side only, which lets researchers trace a maternal line back through many generations.

What is the hollow ball of cells that implants in the wall of the uterus about a week after fertilisation called?

Blastocyst. It takes shape about five days after fertilisation: a fluid-filled ball whose outer shell burrows into the lining, while a small cluster of cells inside goes on to become the baby.

From about the ninth week until birth, the developing baby is known by which name?

A fetus. Before that it is an embryo, and the change of name marks a real milestone: by then every major organ has at least begun to form, so the months that remain are mostly about growing.

Which two small glands, one on each side of the vaginal opening, release a fluid that helps lubricate it?

Bartholin's. The pair is named after a Danish doctor, Caspar Bartholin, and if one of their ducts gets blocked the fluid backs up into a lump known as a Bartholin's cyst.

The uterus is usually described as having what shape?

Pear-shaped, and upside down, with the narrow end pointing down. Before pregnancy it is only about the size of a pear as well, yet its muscular wall stretches to hold a full-term baby.

Which hormone drives the contractions of the uterus during labour?

Oxytocin. Each contraction prompts the release of more of it, a rare self-reinforcing loop that builds until the baby arrives, and doctors give a synthetic version to start or strengthen labour.

What sweeps a released egg along the oviduct toward the uterus?

Cilia. These tiny beating hairs line the duct and set up a current that carries the egg toward the uterus, helped along by gentle squeezes of the muscle in its wall.

When a pregnant woman's waters break, which fluid is being released?

Amniotic fluid. It cushions the baby against knocks and lets it move freely, and the baby swallows and breathes it in as practice for life outside the womb.

After an egg is released, the empty sac that remains in the ovary becomes which yellow, hormone-secreting body?

Corpus luteum. The name is Latin for 'yellow body', after its colour, and it is temporary: if no pregnancy follows it breaks down within a couple of weeks, and a fresh one forms next cycle.

IVF babies were nicknamed 'test-tube babies', but what does the Latin 'in vitro' in IVF literally mean?

In glass. Early experiments on living tissue outside the body were run in glass dishes and tubes, and IVF kept the label because the egg is fertilised in the laboratory before being placed in the uterus.

Around which day of the menstrual cycle is an egg usually released?

Day 14, counting from the first day of a period in a textbook cycle. Cycle lengths vary, mostly in the first half, so the steadier rule is that the egg leaves about two weeks before the next period begins.

Which flower is named from the Greek word for 'testicle', after the shape of its paired root tubers?

Orchid comes from the Greek orkhis, and Middle English was just as blunt: some species were known as bollockwort.

Which erectile dysfunction pill did Pfizer first test as a heart drug for angina?

Viagra did little for angina in early trials, but volunteers reported marked erections, so Pfizer changed course and won approval for it in 1998.

Humans lack the penis bone many mammals have, so what fills erectile tissue to stiffen it instead?

Blood under pressure does all the work in humans, the only great apes to have lost the bone altogether. The few birds with a penis stiffen it with lymph instead.

Which way of delivering a baby was almost always fatal to the mother until the modern era?

A C-section (caesarean) is probably not named for Julius Caesar's own birth: his mother Aurelia lived nearly 50 more years, when the operation killed mothers.

Kate Middleton was hospitalised with hyperemesis gravidarum, a severe form of which pregnancy symptom?

Morning sickness is a misnomer, since the nausea can strike at any hour of the day; it affects about 70–80% of pregnant women.

Which lifeline to an unborn baby is clamped and cut at birth, leaving the belly button behind?

The umbilical cord is about 50 cm long at full term and carries two arteries and one vein, cushioned in a substance called Wharton's jelly.

Carl Linnaeus named the mammals, humans included, after the Latin word for which body part?

Breast is 'mamma' in Latin; Linnaeus coined Mammalia in 1758 for the animals whose mothers feed their young on milk.

Which pregnancy scan was pioneered in 1950s Glasgow using an industrial flaw detector?

Ultrasound was already used in industry to find cracks in welds when Ian Donald turned it on unborn babies; unlike X-rays, it uses no ionising radiation.

Which birth attendant's job title comes from Middle English words meaning 'with woman'?

Midwife names the helper, not the mother: she is the one 'with' the woman in labour. In ancient Greece, midwives had to have given birth themselves.

Which stage of life is confirmed only in hindsight, once twelve months have passed without a period?

Menopause is rare in nature: humans share it with only a few species, among them orcas, belugas and narwhals.

Which latex contraceptive is widely recommended for preventing STIs as well as pregnancy?

Condom material matters: since 1989 the FDA has made lambskin versions carry a warning that they are not for preventing infections, because the skin is porous.

Which animal laid eggs when injected with a pregnant woman's urine, a test used until the 1960s?

A frog from southern Africa, Xenopus, became a living pregnancy test in the 1930s and was exported in great numbers. Earlier versions of the test used mice and rabbits.

Which baby, born in Oldham, England, in 1978, was the first person conceived by IVF?

Louise Brown's younger sister Natalie, also conceived by IVF, later became the first 'test-tube baby' to have a child of her own.

The Tanner scale, devised by a British paediatrician, charts a child's progress through what?

Puberty runs from Tanner stage I, the child's body, to stage V, the adult's. James Tanner built the scale on a two-decade study of girls as they grew up.

Which tube, about five times longer in men than women, carries urine and semen out of the male body?

The urethra's short length in women, about 4 cm against roughly 20 cm in men, is one reason bladder infections are far more common in women.

Which organ grows under the influence of testosterone in a boy's teens, making his voice 'break'?

The larynx grows in both sexes during the teenage years, but most noticeably in boys: the vocal folds lengthen and thicken, and the Adam's apple pushes forward.

Which part of the skeleton is broader in women, long explained as a trade-off between childbirth and easy walking?

The pelvis must fit a roomy birth canal yet still allow efficient walking, a squeeze known as the 'obstetrical dilemma', though recent studies question whether wider hips really cost walkers anything.

Which word describes a woman who carries a baby for others, sometimes with no genetic link to it?

A surrogate can carry an embryo made by IVF from another woman's egg, an arrangement first achieved in 1986.

Which word names both a college academic term and each stage of a human pregnancy?

A trimester of pregnancy lasts roughly 13 weeks. The first carries the highest risk of miscarriage, and the baby's movements are usually first felt midway through the second.

Which operation, depicted in an Egyptian tomb, has been carried out on over a third of all men?

Circumcision removes the foreskin and is among the oldest operations on record: a tomb relief at Saqqara in Egypt shows it being performed around 2300 BCE.

Which vitamin are women advised to take before and during pregnancy to prevent neural tube defects?

Folic acid works best when started before conception, because the neural tube forms in the first month; low levels are blamed for more than half of these defects.

The ovaries and testes also belong to which body system, alongside the thyroid and adrenal glands?

Endocrine glands signal with hormones rather than nerves: the testes secrete testosterone, the thyroid thyroxine and the pineal gland melatonin.

Each of a woman's two ovaries, roughly 3.5 cm long, is about the size and shape of which nut?

Almond is the textbook comparison for an ovary, a solid oval only about 2 cm wide and 1 cm thick.

What is the correct name for the external female genitals, often wrongly called the vagina?

The vulva is everything visible, labia included, while the vagina is only the internal canal; researchers warn that mixing the two up can cause medical and legal confusion.

Which membrane partly covering the vaginal opening takes its name from the Greek for 'thin skin'?

The hymen varies from thin and stretchy to thick and rigid, so its state is no reliable sign of virginity; the WHO has called for 'virginity testing' to end.

Which word describes a baby lying bottom-first in the womb, which doctors may try to turn by hand?

Breech babies can sometimes be turned by firm pressure on the mother's belly, a trick one 17th-century French obstetrician reportedly called a little harder than flipping an omelette.

Which term describes the moment in childbirth when the top of the baby's head first becomes visible?

Crowning, the sign that the birth is close, may be felt by the mother as an intense stretching and burning.

Which 'practice' contractions are named after the English doctor who described them in 1872?

Braxton Hicks tightenings are sporadic rather than regular, and one theory holds that they strengthen the uterine muscle ready for true labour.

Which antibody-rich first milk does a mother produce in the first few days after giving birth?

Colostrum also acts as a mild laxative, helping a newborn pass its first stool. Calves depend on it even more: no antibodies reach them through the placenta, so this first milk supplies their early defences.

Which hormone tells a new mother's body to make milk and even controls the 'crop milk' of pigeons?

Prolactin from the pituitary gland primes the milk glands during pregnancy; in fish, the same hormone is thought to help control salt and water balance.

Which sugar, supplied by glands behind the bladder, makes semen an energy source for swimming sperm?

Fructose-rich fluid makes up about 70% of human semen, so the sperm themselves account for only a small share of the volume.

Which 'friendly' bacteria usually dominate a healthy vagina's microbes and help keep it acidic?

Lactobacilli are known as the gatekeepers of the vaginal ecosystem: the lactic acid they produce is thought to ward off harmful microbes.

Testosterone and other sex hormones are built from which molecule, notorious for clogging arteries?

Cholesterol is the starting material for every steroid hormone, the stress hormone cortisol included, and for vitamin D as well.

Which group of sex hormones, found in both sexes, takes its name from the Greek word for 'man'?

Androgens such as testosterone are made by the ovaries and adrenal glands as well as the testes, so women have them too, just at lower levels.

A man's beard and deeper voice are examples of which class of sex characteristics?

Secondary sex characteristics are linked to sex without being part of the reproductive organs themselves; in the animal world, a peacock's tail is the classic example.

Which term describes the time a baby spends developing in the womb, about 40 weeks in humans?

Gestation normally ends at about 40 weeks, but anything from 37 to 42 weeks is common, and labour that starts before 37 weeks counts as preterm.

Which old diagnosis is named from the Greek for 'uterus', an organ once thought to wander the body?

Hysteria was only dropped from the American psychiatric manual in 1980. Ancient healers treated it by trying to coax the 'wandering' womb back into place with strong smells.

Which word describes a pregnancy growing outside the womb, usually in a tube beside an ovary?

Ectopic pregnancies make up under 2% of pregnancies worldwide, yet they are the most common cause of death among women in early pregnancy.

Which small T-shaped contraceptive is fitted inside the uterus, where it can work for several years?

IUD is short for intrauterine device; the copper kind doubles as the most effective form of emergency contraception when fitted within five days.

Which childbirth instrument did the Chamberlen family of surgeons keep secret for about 150 years?

Forceps let the Chamberlens deliver live babies in difficult births for royal and noble clients; the family's hidden tools turned up in an attic near Maldon in 1813.

Which Delft draper, who took up lenses to inspect his thread, was the first to document sperm cells?

Van Leeuwenhoek made more than 500 lenses and was also first to document bacteria and muscle fibres, yet he refused to show visitors his best microscopes.

Of some 300,000 eggs in a girl's ovaries in her early teens, roughly what number are ever released?

About 400 mature and leave the ovary, around one in each monthly cycle; all the others simply degenerate, a process called atresia.

Which British physiologist won the 2010 Nobel Prize for developing IVF with surgeon Patrick Steptoe?

Robert Edwards could not attend the ceremony, so his wife Ruth, a fellow reproductive scientist and granddaughter of Ernest Rutherford, collected the prize.

Which fringe of projections at the open end of each oviduct helps collect a newly released egg?

Fimbriae form a fringe only about 1 mm wide, yet one of them, the ovarian fimbria, is long enough to reach out and touch the ovary itself.

Which 16-cell stage of the early human embryo is named after the Latin word for 'mulberry'?

The morula reaches the womb three or four days after the egg is fertilised, sealed watertight so that it can later fill with fluid.

Which glycoprotein shell around a human egg must a sperm break through before the two can fuse?

The zona pellucida admits only sperm of the right species and helps make sure that just one of them fertilises the egg.

Which embryonic duct, named after a German embryologist, becomes a male's sperm-carrying tubes?

The Wolffian duct was first described by Caspar Friedrich Wolff in his 1759 dissertation; in female embryos it withers away, though traces can persist.

Which syndrome, also called sympathetic pregnancy, gives some fathers-to-be nausea and weight gain?

Couvade syndrome is named after traditional rituals in which fathers act out pregnancy; men with more symptoms show larger drops in testosterone.

Which cap on the head of a sperm cell holds the enzymes that digest a way into the egg?

The acrosome is built from the cell's Golgi apparatus; in a rare condition where it fails to form, the round-headed sperm that result cause male infertility.

Kisspeptin, which helps switch on sexual maturity, is named after a chocolate from which US town?

Hershey, Pennsylvania, is where the KISS1 gene was isolated in 1996, so the lab named it after the town's famous Hershey's Kisses.