Testicles do drop, but the drop that counts happens before a boy is born, not at puberty.1Children’s Hospital of Philadelphia. Undescended Testes The everyday phrase isn’t entirely wrong, though, because puberty does make them bigger and lower-hanging.2UMass Memorial Health. Puberty: Normal Growth and Development in Boys | UMass Memorial Health The whole trip exists for one practical reason: making sperm needs a spot cooler than the inside of the body.3Sri Lanka Journal of Obstetrics and Gynaecology. A review of scrotal temperature regulation and its importance for male fertility
How Testicles Travel From the Belly to the Scrotum
Testicles first form high in the back of the belly, near the kidneys.4Children’s Hospital of Philadelphia. Undescended Testes Around week 28 of pregnancy, they pass through a channel in the groin wall.5TeachMeAnatomy. Development of the Reproductive System – Embryology – TeachMeAnatomy They usually reach the scrotum by about week 33, though the overall journey spans weeks 28 to 40.6TeachMeAnatomy. Development of the Reproductive System – Embryology – TeachMeAnatomy7Children’s Hospital of Philadelphia. Undescended Testes Each testicle keeps its original blood vessels, which still branch off near the kidneys, so those vessels lengthen along the way.8TeachMeAnatomy. Development of the Reproductive System – Embryology – TeachMeAnatomy
The trip works in two stages. In the first, the body grows up and away while the testicle stays low. The second stage is the real move, from the groin down into the scrotum. Each stage needs a different hormone signal.
Each testicle is tied to the scrotum by a soft, jelly-like cord.9TeachMeAnatomy. Development of the Reproductive System – Embryology – TeachMeAnatomy10PubMed Central. The gubernaculum during testicular descent in the human fetus Before descent, this cord swells, lengthens and fills with water, while the testicle itself stays the same size.11PubMed Central. The gubernaculum during testicular descent in the human fetus So the cord’s growth, not a push from the testicle, does most of the work of widening the path through the groin.12PubMed Central. The gubernaculum during testicular descent in the human fetus In the final stage, the cord actively grows out of the belly wall toward the scrotum, drawing the testicle with it.13PubMed Central. The role of the gubernaculum in the descent and undescent of the testis
The testicles steer their own trip by releasing hormones.14PubMed Central. New Insights into the Role of INSL-3 in the Development of Cryptorchidism One hormone, INSL3, thickens the cord so the testicle stays anchored low while the kidneys grow away upward.15PubMed Central. Expression and Role of INSL3 in the Fetal Testis Testosterone then drives the second stage, from the groin into the scrotum.16PubMed Central. Hypogonadism and Cryptorchidism The importance of INSL3 shows in mice lacking it, which end up with both testicles stuck inside and are infertile.17PubMed. Cryptorchidism in mice mutant for Insl3
The trip also leaves a passage through the groin wall, which normally seals off after the testicle has passed through. If it doesn’t close fully, a loop of intestine or fluid can slip into the opening, which is how a common type of hernia in infants forms. That’s why undescended testicles and groin hernias in baby boys often turn up together.
What Actually Happens at Puberty
At the start of puberty, the testicles get bigger and hang lower.18UMass Memorial Health. Puberty: Normal Growth and Development in Boys | UMass Memorial Health The scrotum also darkens, gets looser and develops small bumps.19UMass Memorial Health. Puberty: Normal Growth and Development in Boys | UMass Memorial Health But in most boys the testicles have been in the scrotum since infancy, so nothing new is arriving. A teen who notices them hanging lower is seeing normal growth, often one of the first signs of puberty. The myth stuck because voice deepening and other changes get lumped together as “dropping,” and the real descent happened before anyone’s memory.
Testicle growth often comes first, and the deeper voice, hair growth and growth spurt come later. The enlargement of the testicles is driven by the same hormonal wake-up that starts sperm production.
Some things are normal along the way. One testicle usually hangs a bit lower than the other, and small size differences between sides are common. A testicle that was never felt in the scrotum is worth a doctor’s check at any age.
When a Baby’s Testicle Hasn’t Come Down
About 3% of full-term baby boys are born with one or both testicles not yet in the scrotum, a condition doctors call an undescended testicle or cryptorchidism.20Harvard Health. Undescended Testicle (Cryptorchidism) In premature babies the figure is about 30%, because they were born before the trip usually finishes.21Harvard Health. Undescended Testicle (Cryptorchidism) Babies small for their age at birth face a slightly higher risk even when born on time.22American Urological Association. Evaluation and Treatment of Cryptorchidism One proposed reason is less hormone signaling before birth. Some research also links mothers’ exposure to certain chemicals, such as some pesticides, during pregnancy with somewhat higher risk.23PubMed Central. Human Exposure to Endocrine-Disrupting Chemicals and Prenatal Risk Factors for Cryptorchidism and Hypospadias: A Nested Case–Control Study24PubMed Central. The Association between Maternal Endocrine-Disrupting Chemical Exposure during Pregnancy and the Incidence of Male Urogenital Defects: A Systematic Review and Meta-Analysis
The chemical research shows an association only; it doesn’t prove that any one exposure caused a particular baby’s condition. Most boys with an undescended testicle have mothers with no known exposure, and parents shouldn’t blame themselves.
Most undescended testicles drop by themselves in the first 3 to 6 months.25Harvard Health. Undescended Testicle (Cryptorchidism) By 6 months, fewer than 1% of boys still have one.26Harvard Health. Undescended Testicle (Cryptorchidism) For preemies, doctors count those 6 months from the due date, not the birth date.27Journal of Urology. Evaluation and Treatment of Cryptorchidism
A parent may notice that the scrotum on one side looks small or flat. Doctors check at newborn and well-baby visits. For the exam, the baby lies on his back with legs in a frog position to relax the muscles, and the doctor feels along the groin path and tries to ease the testicle down. Waiting until 6 months is the normal plan, not neglect. If it hasn’t come down by then, ask for a referral to a pediatric urologist or surgeon.
Why Testicles Move Up and Down
A thin muscle around each testicle pulls it toward the body in cold or when touched. The cold response makes sense: in experiments with bulls, cold water dropped temperature inside the testicles by as much as 9 to 12 °C.28Zentralblatt für Veterinärmedizin. Some Observations on the Rectal, Scrotal, Sub‐Scrotal and Endo‐Testicular Temperatures in the Bull Pulling up close to the body protects them from getting too cold.29Zentralblatt für Veterinärmedizin. Some Observations on the Rectal, Scrotal, Sub‐Scrotal and Endo‐Testicular Temperatures in the Bull
Touching the inner thigh can also trigger it, which doctors use as a normal reflex test. In young boys the reflex can be especially brisk, which is why a testicle can seem to disappear. Adults notice it too: cold swims, fear and exercise lift the testicles, and heat loosens them to hang lower.
A retractile testicle is one that sometimes sits high but can be gently brought to the bottom of the scrotum and stays a while.30PubMed Central. Comparison of diagnostic and treatment guidelines for undescended testis It has finished its journey and is usually considered normal.31PubMed Central. Comparison of diagnostic and treatment guidelines for undescended testis Still, up to a third of retractile testicles later move back up and stay up,32PubMed Central. Comparison of diagnostic and treatment guidelines for undescended testis so a retractile testicle is worth rechecking at routine visits. Parents can check in a warm bath with the child calm, which gives a truer picture because the muscle is relaxed.
Why Testicles Sit Outside the Body
Testicles stay about 2 to 3 °C cooler than the rest of the body.33PubMed. The process of spermatogenesis liberates significant heat and the scrotum has a role in body thermoregulation Healthy sperm production needs that slightly lower temperature,34Sri Lanka Journal of Obstetrics and Gynaecology. A review of scrotal temperature regulation and its importance for male fertility so the body has several ways to keep the scrotum cool.35Sri Lanka Journal of Obstetrics and Gynaecology. A review of scrotal temperature regulation and its importance for male fertility A testicle left inside the warm belly can’t make sperm properly.
The scrotal skin loosens when it is warm, letting the testicles hang away from the body, and tightens when cool, drawing them closer to conserve warmth. Long hot baths or a laptop on the lap add heat, which is why couples trying to conceive hear advice about it. The effect is usually temporary.
Not all mammals do this. Many, including elephants and the platypus and its relatives, keep their testicles inside.36PubMed. The evolutionary history of testicular externalization and the origin of the scrotum The scrotum appears to have evolved at least twice in mammal history.37PubMed. The evolutionary history of testicular externalization and the origin of the scrotum
Treatment When a Testicle Stays Up
The standard fix is a surgery called orchidopexy, which brings the testicle down and fixes it in the scrotum.38Journal of Urology. Evaluation and Treatment of Cryptorchidism US guidelines advise surgery within a year after 6 months of age; European guidelines say by 12 to 18 months.39PubMed Central. Are we still too late? Timing of orchidopexy40Journal of Urology. Evaluation and Treatment of Cryptorchidism Timing matters because boys operated on at 9 months showed catch-up growth of the testicle, while those operated on at 3 years didn’t.41PubMed. Surgical treatment of unilaterally undescended testes: testicular growth after randomization to orchiopexy at age 9 months or 3 years The 6-month mark is chosen because the chance of a spontaneous drop after that is very small. Surgery succeeds about 90% of the time.42PubMed Central. Epidemiology, Classification and Management of Undescended Testes: Does Medication Have Value in its Treatment?
The surgeon frees the testicle and its vessels from surrounding tissue so they are long enough to reach the scrotum. A small pocket is made in the scrotum, and the testicle is stitched in so it can’t slip back up. If the testicle can be felt in the groin, surgery goes through a small cut in the groin or scrotum,43Journal of Urology. Evaluation and Treatment of Cryptorchidism and the operation often takes under an hour. If it can’t be felt, the surgeon re-examines under anesthesia and, if needed, looks inside the belly.44Journal of Urology. Evaluation and Treatment of Cryptorchidism
It’s usually an outpatient procedure under general anesthesia. Most children return to normal play within a week or two, and follow-up checks confirm the testicle stays down.
Hormone shots are a less effective option. In careful studies, they bring the testicle down in only about 1 in 5 boys.45PubMed Central. Epidemiology, Classification and Management of Undescended Testes: Does Medication Have Value in its Treatment? Nordic experts advise against them, citing poor results and possible harm to later sperm production.46PubMed. Nordic consensus on treatment of undescended testes
Long-Term Effects on Fertility and Cancer Risk
About two-thirds of men with one undescended testicle go on to father children, versus fewer than a third with both.47PubMed Central. Undescended testicle: An update on fertility in cryptorchid men Changes in the testicle start in the first months of life, even with timely surgery.48PubMed Central. The Undescended Testis: Diagnosis, Treatment and Long-Term Consequences They can begin in the sperm-making cells before surgery is done, so surgery protects what can be protected but can’t always undo early changes. One long-term follow-up found no clear fertility gain from earlier surgery.49PubMed. Fertility potential in men with a history of congenital undescended testes: a long-term follow-up study With one side affected, the other testicle can usually still make sperm, which helps explain the better odds. With both sides affected, both may have been compromised.
A history of undescended testicle raises testicular cancer risk about 4-fold overall.50PubMed Central. Epidemiology and risk factors of testicular cancer: a systematic review Surgery before 13 cut the added risk roughly in half compared with surgery later.51PubMed. Age at surgery for undescended testis and risk of testicular cancer Risk is highest when both sides were affected and in the testicle that was undescended.52PubMed Central. Epidemiology and risk factors of testicular cancer: a systematic review Undescended testicles also carry a higher risk of twisting and hernia.53Cigna. Undescended testicle (cryptorchidism) Testicular cancer is uncommon even with a raised risk, and it is very treatable when found early.
Adult men can learn to do a monthly self-check in the shower, and should tell doctors about the history even if it was treated in babyhood. A lump, swelling or heaviness deserves a prompt visit. Sudden severe pain needs emergency care because of the twisting risk.