Testicular descent happens before birth, not during puberty or childhood as many people assume. In a developing fetus, the testes form near the kidneys and migrate downward through the abdomen, passing through the inguinal canal and into the scrotum typically between about 24 and 36 weeks of pregnancy. By the time a full-term baby boy is born, both testes have usually completed the journey. When they haven’t, the condition is called cryptorchidism, and it triggers a medical timeline that matters for fertility and long-term health.
How Descent Works Before Birth
Early in fetal development, the testes sit behind the lining of the abdominal cavity, roughly at the level of the lower back. They begin their descent in two phases. The first phase pulls them from the abdomen down toward the groin, guided largely by changes in a structure called the gubernaculum, a cord of tissue that connects the developing testis to the scrotum. The second phase moves them through the inguinal canal and into the scrotum itself. Hormones drive both stages, with a combination of testosterone and a hormone called insulin-like hormone 3 (INSL3) playing key roles in the first phase, while androgens and nerve signaling dominate the second.1African Journal of Urology. The curious case of testicular descent: factors controlling testicular descent with a note on cryptorchidism
A study examining human fetal specimens found that descent was first observed around 24 weeks of intrauterine life, with complete bilateral descent achieved by 36 weeks.2Journal of the Anatomical Society of India. The testicular position and descent in developing human Another study looking at over 200 fetuses found that the earliest inguinal position (midway through the journey) appeared at about 17 weeks post-conception, and every fetus past 30 weeks had both testes fully in the scrotum.3PubMed. Inguinoscrotal stage of testicular descent: analysis in 217 human fetuses The passage through the inguinal canal itself tends to be rapid. In the majority of cases, both sides descend symmetrically, though when asymmetry occurs, the left side tends to lead.
This prenatal timeline is why “dropping” is essentially a done deal by the time a boy is born at term. The phrase “when do they drop” implies something that happens during childhood or adolescence, but the truth is that if descent hasn’t occurred by full-term birth, it’s already behind schedule.
When Testes Haven’t Descended at Birth
Cryptorchidism, meaning one or both testes haven’t reached the scrotum, is one of the most common genital conditions in newborn boys. Birth rates range from roughly 2 to 9 percent depending on the population studied, with premature infants at considerably higher risk because their testes simply haven’t had time to finish the trip.4PubMed. Genetics of cryptorchidism and testicular regression The causes are complex and can involve genetic factors, hormonal disruptions, environmental exposures, and maternal health during pregnancy.
There is a window during early infancy when spontaneous descent can still happen. A hormonal surge in the first few months of life, sometimes called “mini-puberty,” produces a spike in LH and testosterone that can push a hanging testis the rest of the way down. One study found that about 46 percent of undescended testes completed their descent between 3 and 6 months of age, though the rate was higher in premature babies (about 63 percent) than in full-term babies (about 43 percent).5PubMed. Testicular descent: when to interfere? In full-term boys, no spontaneous descent was observed after 4 months, while in preterm boys it could continue up to 6 months.
After this early window closes, the odds of a testis descending on its own become very low. A study re-examining boys with cryptorchidism at age one or beyond found that only about 7 percent experienced spontaneous descent, and every one of those boys had originally been diagnosed before 6 months of age. Boys first identified with the condition after 6 months never had spontaneous descent.6PubMed. What is the rate of spontaneous testicular descent in infants with cryptorchidism? This sharp cutoff is one reason pediatric guidelines push for early action rather than prolonged watchful waiting.
Retractile Testes and the Confusion They Cause
A common source of parental anxiety is the retractile testis, where a testis that has descended normally gets temporarily pulled up out of the scrotum by the cremasteric reflex, a muscle contraction triggered by cold, touch, or anxiety. The testis can be gently guided back into the scrotum during a physical exam, and it stays there at least briefly. This is considered a variant of normal anatomy and doesn’t require surgery.7PubMed Central. Could miRNAs be used as markers for distinguishing undescended testes from retractile testes? A prospective controlled pilot/exploratory study
The distinction matters because a truly undescended testis cannot be manually brought down into the scrotum, or if it can be coaxed there, it springs right back up. A retractile testis, by contrast, descends easily and rests comfortably in the scrotum when the cremasteric reflex isn’t active. Research into the cremasteric reflex in boys with undescended testes has found that the reflex tends to be hyperactive in those cases, with faster triggering and more prolonged muscle contraction, which may have contributed to preventing normal descent in the first place.8PubMed. Cremasteric reflexes of boys with descended, retractile, or undescended testes: an electrophysiological evaluation
Parents often notice the retractile phenomenon during bath time or diaper changes and worry that a testis has “gone back up.” In most cases, if a doctor has previously confirmed both testes in the scrotum, the retractile testis is harmless and will stop retracting as the boy grows and the cremasteric reflex becomes less active around puberty. That said, retractile testes do need monitoring, because some can become “trapped” in the inguinal area over time, a situation described below.
Acquired Undescended Testis: When They Go Back Up
One of the less intuitive findings in pediatric urology is that a testis which has verifiably descended into the scrotum can later ascend back into the inguinal canal and stay there. This is called an acquired undescended testis, and it’s distinct from a retractile testis in that it doesn’t come back down during a physical exam. A study looking at over 300 boys referred for undescended testes found that in about 44 percent of cases, there was documentation that the testes had previously been in a normal position. Among those with detailed records, the median age when ascent was estimated to have occurred was about 6 years, with the 75th percentile at roughly 7 years and the 95th percentile at about 11 years.9PubMed. Acquired undescended testis: When does the ascent occur?
This means a boy whose testes were confirmed normal at his infant checkups can develop what looks and acts like cryptorchidism during elementary school. The mechanism isn’t fully understood, but it appears to involve a failure of the spermatic cord to elongate in proportion to the child’s growth, effectively pulling the testis upward as the body gets taller. Acquired undescended testes need the same surgical evaluation as those present from birth, because they carry similar risks to fertility and may require the same corrective procedure.
This finding has changed how pediatricians approach well-child visits. Checking testicular position isn’t just an infancy task; it should be part of routine exams throughout childhood. A boy who was fine at age two can have a problem at age seven, and catching it early still matters for outcomes.
Why the Timing of Surgery Matters
When spontaneous descent doesn’t happen by 6 months of age (adjusted for prematurity), current guidelines from the American Urological Association recommend surgery within the next year, meaning the procedure should ideally happen before the boy’s first birthday.10American Urological Association. Evaluation and Treatment of Cryptorchidism The reasoning is straightforward: the longer a testis stays in the wrong position, the more damage it sustains. The fertility index, defined as the number of sperm-producing cells per tubular cross-section, begins to decline after the first year of life in undescended testes, and the loss accelerates with time.
Despite these guidelines, many boys still get surgery later than recommended. A large study found that the median age at surgery was about 17 months, and among boys having surgery before age three, roughly two-thirds had their operation after the recommended 12-month cutoff. Boys from socioeconomically disadvantaged backgrounds, those living in rural areas, and those without private insurance were more likely to have delayed surgery.11Pediatrics. Age at Surgery and Outcomes of an Undescended Testis The good news is that the trend has been moving in the right direction, with median surgical ages falling from about 21 months for boys born in 2001 to 13 months for those born in 2010.
The standard surgery is called orchiopexy, where the testis is moved into the scrotum and secured there. For testes that can be felt during examination, primary orchiopexy has a success rate of about 96 percent. For testes that can’t be felt, meaning they may be deep in the abdomen, the surgeon typically starts with a diagnostic laparoscopy to locate the testis and then performs a one- or two-stage procedure to bring it down, with success rates of roughly 79 to 86 percent depending on the approach.12PubMed Central. Effectiveness of hormonal and surgical therapies for cryptorchidism: a systematic review
Hormonal therapy, using injections of hormones like hCG to stimulate descent, has been tried for decades but is no longer recommended. The evidence shows that descent rates with hormonal treatment barely exceed those seen with placebo, and there is no good evidence that the effects last.10American Urological Association. Evaluation and Treatment of Cryptorchidism
Why Imaging Often Doesn’t Help
Parents sometimes expect that an ultrasound will quickly settle whether a testis is truly undescended and where it is. In practice, ultrasound is surprisingly unhelpful for the most concerning scenario: a testis that can’t be felt on physical exam. A systematic review found that ultrasound has a sensitivity of only about 45 percent and a specificity of about 78 percent for locating nonpalpable testes. A positive ultrasound only shifts the probability that the testis is in the abdomen from 55 to 64 percent, which isn’t enough to change what happens next.13PubMed Central. Diagnostic performance of ultrasound in nonpalpable cryptorchidism: a systematic review and meta-analysis
Whether the ultrasound finds the testis or doesn’t, the next step is the same: surgical exploration, usually laparoscopy. The laparoscope can definitively locate the testis and allow the surgeon to fix the problem in the same session. MRI is better than ultrasound at locating testes, but it’s expensive, not always available, and usually requires sedation in young children.14PubMed Central. Diagnostic Imaging in Cryptorchidism: Utility, Indications, and Effectiveness Most experts argue that skipping the ultrasound altogether saves time, cost, and false reassurance without changing outcomes. The most reliable diagnostic tool for a nonpalpable testis is the hands of an experienced examiner, followed by the laparoscope.
Fertility Consequences
The scrotum exists to keep testes a few degrees cooler than core body temperature, and sperm production is sensitive to heat. A testis stuck in the abdomen or inguinal canal is chronically too warm, and this damages the cells responsible for making sperm. The problem starts earlier than many people realize. Even in infancy, a critical transformation occurs in which fetal germ cells convert into the cell type that will eventually produce sperm in adulthood. In cryptorchid testes, this transformation is impaired, and the impairment is now understood to be a primary cause of reduced fertility later in life.15PubMed Central. Undescended testicle: An update on fertility in cryptorchid men
Bilateral cryptorchidism (both sides) carries a more significant fertility impact than unilateral (one side). Men who had one undescended testis corrected in childhood generally have paternity rates close to the general population, though their sperm counts may be somewhat lower. Men with uncorrected bilateral cryptorchidism face much higher rates of infertility.16PubMed Central. Cryptorchidism and its impact on male fertility: a state of art review of current literature Early orchiopexy doesn’t guarantee normal fertility, but it preserves the best chance by halting ongoing germ cell loss. The AUA guidelines emphasize that uncorrected testes, especially those that can’t be felt on exam, suffer continuing loss of both sperm-producing cells and hormone-producing cells the longer they remain out of position.10American Urological Association. Evaluation and Treatment of Cryptorchidism
Cancer Risk and the Age-at-Surgery Effect
Men with a history of cryptorchidism face an elevated risk of testicular cancer compared to the general population, but the size of that risk depends heavily on when it was corrected. A large Swedish study found that boys who had orchiopexy before age 13 had about twice the risk of testicular cancer compared to the general population, while those treated at age 13 or older had more than five times the risk.17PubMed. Age at surgery for undescended testis and risk of testicular cancer A review of the broader literature puts the overall relative risk at somewhere between about 3 and 8, with prepubertal correction roughly halving the cancer risk compared to late or no correction.18PubMed. Cryptorchidism and testicular cancer: separating fact from fiction
A few nuances are worth knowing. Only the undescended testis carries the elevated risk; the opposite testis, if it descended normally, does not face an increased chance of cancer. The type of cancer also differs depending on whether the testis was corrected: testes that remain undescended are more likely to develop seminoma, while testes that were surgically corrected are more likely to develop nonseminoma if cancer occurs.18PubMed. Cryptorchidism and testicular cancer: separating fact from fiction Testes that are completely intra-abdominal carry the highest risk of all, with one institution’s experience suggesting the cancer risk in an ectopic testis may be up to 40 times that of a normally positioned one.19PubMed Central. Malignancy in an Undescended Intra-abdominal Testis: a Single Institution Experience
Even after successful orchiopexy, men with a history of cryptorchidism are typically advised to perform regular testicular self-exams throughout adulthood. Surgery reduces the risk but doesn’t eliminate it entirely, and having the testis in the scrotum at least makes self-examination and clinical monitoring straightforward.
Why Testes Are External in the First Place
The whole process of testicular descent exists because sperm production works best at a temperature a few degrees below the body’s core. This constraint has shaped mammalian anatomy for a remarkably long time. Researchers analyzing the evolutionary history of testicular position across the mammalian family tree have proposed that the scrotum evolved at least twice independently, once in marsupials and once in the group of placental mammals that includes humans.20PubMed. The evolutionary history of testicular externalization and the origin of the scrotum In the ancestral state, testes were internal, and many mammals have since returned to internal testes when they evolved other ways to keep sperm cool or when their reproductive biology changed enough that the temperature issue became less critical.
Elephants, for instance, keep their testes inside the abdomen, as do sloths, armadillos, and several marine mammals. The prevailing theory is that descent into a scrotum is a biologically expensive solution to temperature regulation (it exposes the gonads to injury and requires a complex developmental migration), so it gets lost evolutionarily whenever an alternative cooling mechanism or a reduced temperature sensitivity evolves.21PubMed. The evolution of the scrotum and testicular descent in mammals: a phylogenetic view Humans, lacking any such alternative, are stuck with the elaborate prenatal descent process and the occasional complications that come with it.
Psychological Dimensions of Late Diagnosis
Most discussions of cryptorchidism focus on the physical risks, but the psychological dimension can be real for older boys and adolescents diagnosed late. Research into the psychopathological aspects of cryptorchidism in children and adolescents has identified several factors that influence psychological vulnerability: the age at which the condition is diagnosed and treated, the restoration of normal genital appearance, the child’s personality stability, and family dynamics all play a role in how the experience affects the child emotionally.22PubMed. Psychopathological aspects of cryptorchidism in children and adolescents
A boy who has corrective surgery at 10 months is unlikely to remember it. A boy who discovers at age 12 that a testis is missing from his scrotum faces a very different experience, one involving body image concerns during an already self-conscious developmental period. This is another reason early detection and correction matter beyond the biological arguments about fertility and cancer: catching and fixing the problem during infancy spares the child from ever having to process it as an identity-relevant event. For boys diagnosed later, including those with acquired undescended testes discovered during school-age checkups, clear communication from the medical team and parents about the straightforward nature of the correction can make a meaningful difference in how the child copes.