Testicular descent is almost entirely a prenatal event, not something that happens during puberty. In a typical pregnancy, the testes form near the kidneys and migrate downward in two distinct phases, reaching the scrotum by around the seventh or eighth month of gestation. The popular idea that “balls drop” during adolescence confuses testicular growth and scrotal changes at puberty with the actual descent, which happened years earlier. When descent does not complete on schedule, the condition is called cryptorchidism, and it carries real consequences for fertility and cancer risk if left untreated.
Two Phases of Descent Before Birth
The testes begin forming high in the abdomen, near the developing kidneys, during the first weeks of embryonic life. Their journey to the scrotum unfolds in two stages. The first phase, sometimes called the transabdominal phase, occurs roughly during the second trimester. During this window, the testes move from their original position toward the internal inguinal ring, the gateway between the abdomen and the groin. A study of human fetuses between 13 and 23 weeks found that the testes stayed adjacent to the internal ring throughout that entire period, suggesting the transabdominal portion is more about anchoring in place than dramatic travel.1PubMed Central. Is there a trans-abdominal testicular descent during the second gestational trimester? Study in human fetuses between 13 and 23 weeks post conception The key structure responsible for holding the testis near the groin is the gubernaculum, a ligament that enlarges and essentially prevents the testis from drifting upward as the rest of the abdomen grows around it.2PubMed. Development and descent of the testis in relation to cryptorchidism
The second phase, the inguinoscrotal phase, is the one that actually moves the testis through the inguinal canal and into the scrotum. This happens during the third trimester, typically between weeks 25 and 35. The gubernaculum develops almost limb-bud-like properties, actively migrating across the pubic region to guide the testis into the scrotum.3Endocrine Reviews. The Regulation of Testicular Descent and the Effects of Cryptorchidism Because this second phase happens so late in pregnancy, babies born prematurely are far more likely to have undescended testes simply because the process did not have time to finish.
What Drives the Descent
Two hormones produced by the fetal testes orchestrate the process. The first phase depends heavily on a hormone called INSL3, which is produced by Leydig cells in the testis and signals the gubernaculum to enlarge. The second phase is driven primarily by testosterone, which triggers the gubernaculum to remodel and pull the testis through the inguinal canal.4PubMed. Testicular descent: INSL3, testosterone, genes and the intrauterine milieu If either hormone is insufficient or if the receptors that respond to them are not working properly, descent can stall partway. This hormonal dependence is also why conditions that disrupt fetal hormone levels, whether genetic or environmental, can lead to undescended testes.
When Testes Are Not in the Scrotum at Birth
Undescended testes are one of the most common findings in newborn boys, especially among those born early.5PubMed Central. Current Management of Undescended Testes Estimates vary, but roughly 2 to 5 percent of full-term boys and up to 30 percent of premature boys have at least one undescended testis at birth. Several factors raise the risk. Preterm birth, low birth weight, and restricted growth in the womb are among the strongest predictors.6Frontiers in Endocrinology. The epidemiology of cryptorchidism and potential risk factors, including endocrine disrupting chemicals Family history of the condition roughly quadruples the odds. Other factors identified in research include low birth weight and certain maternal exposures during pregnancy, such as analgesic use.7PubMed. Risk factors for cryptorchidism: a nested case-control study
The good news is that many of these cases resolve on their own during the first few months of life. A brief hormonal surge occurs in baby boys around two to three months of age, during which luteinizing hormone and testosterone spike before returning to baseline by about six months. This surge can nudge an undescended testis the rest of the way down. In one study tracking infants with palpable undescended testes, nearly half of the testes descended spontaneously between three and six months of age, closely mirroring that postnatal hormone spike.8PubMed. Testicular descent: when to interfere? Full-term boys who experienced spontaneous descent did so by the fourth month, while preterm boys sometimes had descent as late as six months. After that window closes, the chances of spontaneous descent drop sharply. One study found that spontaneous descent occurred in only about 7 percent of boys re-examined at age one or beyond, and every one of those boys had first presented before six months of age.9PubMed. What is the rate of spontaneous testicular descent in infants with cryptorchidism?
Why They Sit Outside the Body
The scrotum exists for temperature regulation. Sperm production is sensitive to heat, and the testes need to stay a few degrees cooler than core body temperature to produce sperm normally. Research puts that ideal temperature gap at roughly 2 to 6 degrees Celsius below body temperature.10PubMed. The effects and molecular mechanism of heat stress on spermatogenesis and the mitigation measures When temperatures rise, the consequences cascade: sperm DNA integrity breaks down, abnormal sperm forms increase, and germ cells die off at higher rates.11Reproductive BioMedicine Online. Causes, effects and molecular mechanisms of testicular heat stress This is why a testis that remains inside the body, as in cryptorchidism, faces progressive damage over time.12PubMed Central. Responses and coping methods of different testicular cell types to heat stress: overview and perspectives
From an evolutionary standpoint, the scrotum likely developed alongside the evolution of warm-bloodedness in the mammalian lineage. The “cooling hypothesis,” first proposed nearly a century ago, argues that as mammals evolved higher and more stable body temperatures, they needed a way to keep sperm production viable. A scrotal location provided that cooler environment and may also have lowered the rate of spontaneous genetic mutations in sperm.13PubMed Central. Reappraising the exteriorization of the mammalian testes through evolutionary physiology Some mammals, like elephants and certain marine species, have since lost the scrotum and developed alternative cooling strategies, suggesting that testicular descent is costly and gets dropped whenever a workaround is available.14Journal of Theoretical Biology. The Evolution of the Scrotum and Testicular Descent in Mammals: a Phylogenetic View
The Puberty Misconception
Many people believe that “balls drop” during puberty. What actually happens at puberty is that the testes grow substantially in size, the scrotum elongates and darkens, and the whole package becomes more visible and more pendulous. The testes are already in the scrotum; they are just getting bigger. In longitudinal studies tracking testicular volume, the marker of pubertal onset is a testicular volume of about 4 milliliters, which the median boy reaches around age 11 to 12. The range is wide: some boys hit that mark as early as age 10 and others not until nearly 14.15PubMed Central. Pubertal testicular volume references for ruler, orchidometer, and ultrasonography measurements based on a longitudinal follow-up By the end of puberty, a typical adult testis reaches 15 to 25 milliliters, several times its prepubertal size. This rapid growth, combined with thinner scrotal skin and longer spermatic cords, creates the sensation and appearance that the testes have “dropped,” but they have not descended in the anatomical sense. They simply grew up where they already were.
Retractile Testes and Ascending Testes
Parents and pediatricians sometimes encounter a scenario where a testis seems to be in the scrotum one day and gone the next. This can be one of two things, and telling them apart matters. A retractile testis is one that has descended normally but gets pulled up into the inguinal canal by the cremasteric reflex, a muscle contraction triggered by cold, touch, or anxiety. You can usually coax a retractile testis back into the scrotum with gentle pressure, and it stays put once there. No treatment is needed. Electrophysiological studies of the cremaster reflex in boys with retractile testes have not conclusively shown that the reflex is overactive compared to normal, so the exact mechanism remains debated.16PubMed. Cremasteric reflexes of boys with descended, retractile, or undescended testes: an electrophysiological evaluation
More concerning is acquired cryptorchidism, sometimes called ascending testis. In these cases a testis that was clearly in the scrotum at birth gradually moves back up and becomes genuinely undescended. This is surprisingly common. A Danish study found that ascending testes accounted for more than half of all cryptorchid testes seen in childhood by 18 months and over two-thirds by age three.17PubMed. Acquired cryptorchidism is frequent in infancy and childhood This means that a clean bill of health at birth does not guarantee the testes will stay put. Pediatricians generally check testicular position at every well-child visit for this reason, and parents should know that it is worth mentioning if a testicle seems harder to find during bath time than it used to be.
Treatment When Descent Does Not Happen
If a testis has not descended on its own by six months of age, the window for spontaneous resolution is essentially closed. At that point, treatment comes into play. There are two main options: hormonal therapy and surgery.
Hormonal therapy, usually with injections of human chorionic gonadotropin (hCG), attempts to stimulate the same testosterone-driven mechanism that should have completed descent in the womb. In one series of over 100 treated testes, about 45 percent descended fully to the scrotum after hCG therapy, and the treatment moved roughly 80 percent of testes at least one level closer to the scrotum even when full descent was not achieved.18PubMed Central. Neoadjuvant human Chorionic Gonadotropin (hCG) therapy may improve the position of undescended testis: a preliminary report However, there is a real split in clinical opinion on this. Some guidelines recommend hormone therapy as a reasonable first step, with surgery to follow if hormones fail.19PubMed Central. The Undescended Testis: Diagnosis, Treatment and Long-Term Consequences Other guidelines, particularly more recent ones, do not recommend routine hormonal therapy for undescended testes, citing a lack of strong evidence that it improves long-term outcomes compared to going straight to surgery.20PubMed Central. Comparison of diagnostic and treatment guidelines for undescended testis
Surgery, called orchidopexy, is the definitive treatment. The procedure brings the testis into the scrotum and fixes it in place. Current guidelines generally recommend orchidopexy between 6 and 12 months of age, though practice varies. A survey of surgical consultants found the median preferred age was about 10 to 11 months, with pediatric urologists tending to favor operating a bit earlier (around 9 months) than general surgeons (around 12 months).21The Bulletin of the Royal College of Surgeons of England. Consultants’ attitudes to the ideal age for orchidopexy Despite these recommendations, many boys still undergo surgery later than ideal. An audit found that only about 15 percent of hospital patients had orchidopexy before their first birthday, while more than 20 percent were between three and five years old, and nearly a quarter were six or older.22PubMed Central. Are we still too late? Timing of orchidopexy This matters because outcomes worsen with delay.
Cancer Risk and Fertility
The longer a testis sits outside the scrotum, the more damage accumulates. An undescended testis is a well-established risk factor for testicular cancer, and the risk climbs with delay in correction. One large study found a relative risk of about 3.7 for testicular cancer among men with a history of an undescended testis, with the risk highest in men whose cryptorchidism was never corrected.23PubMed. Testicular cancer risk among young men: role of cryptorchidism and inguinal hernia For an intra-abdominal testis that was never brought down, the cancer risk has been reported as up to 40 times higher than a normally positioned testis.24PubMed Central. Malignancy in an Undescended Intra-abdominal Testis: a Single Institution Experience The relationship between the age at surgical correction and cancer risk remains an active area of research, but the consensus is that earlier surgery reduces, though may not fully eliminate, the elevated risk.25PubMed. Age at surgery for undescended testis and risk of testicular cancer
Fertility is also affected. Heat damage to the germ cells that produce sperm begins early. Even when only one testis was undescended, paternity rates in adulthood are reported at roughly two-thirds, compared to the general population. When both testes were undescended, the rate drops to less than one-third.26PubMed Central. Undescended testicle: An update on fertility in cryptorchid men These numbers reinforce why timely treatment matters so much, and why pediatricians push for orchidopexy within the first year of life whenever spontaneous descent does not occur.
Finding a Testis That Cannot Be Felt
When a doctor cannot feel a testis during a physical exam, the question becomes whether it is hiding somewhere inside the body or absent entirely. Imaging can help, but the results are uneven. Standard ultrasound is good at confirming testes that are already palpable but struggles with nonpalpable ones, achieving a sensitivity of only about 45 percent for locating them. Because of this poor detection rate, ultrasound is not considered useful in the routine workup of boys with cryptorchidism.27PubMed Central. Diagnostic Imaging in Cryptorchidism: Utility, Indications, and Effectiveness MRI performs better, and combining standard MRI with specialized diffusion-weighted imaging raises accuracy into the mid-90s percent range.28The Egyptian Journal of Radiology and Nuclear Medicine. Magnetic resonance imaging for detection of non palpable undescended testes: Diagnostic accuracy of diffusion-weighted MRI in comparison with laparoscopic findings Still, the gold standard remains diagnostic laparoscopy, a minimally invasive surgical procedure that can both find and fix the problem in the same session, with nearly perfect sensitivity.27PubMed Central. Diagnostic Imaging in Cryptorchidism: Utility, Indications, and Effectiveness
Environmental Chemicals and Rising Rates
There is growing concern that exposure to certain chemicals during pregnancy could contribute to cryptorchidism. A systematic review and meta-analysis found a significant association between maternal exposure to endocrine-disrupting chemicals and an increased risk of cryptorchidism, with pesticides and PCBs showing the strongest links.29PubMed Central. The Association between Maternal Endocrine-Disrupting Chemical Exposure during Pregnancy and the Incidence of Male Urogenital Defects: A Systematic Review and Meta-Analysis Certain phthalates, chemicals widely used in plastics and personal care products, have also been linked to higher risk.30The Turkish Journal of Pediatrics. Maternal phthalate exposure during pregnancy and male reproductive disorders: a systematic review and metaanalysis The evidence here is still developing, and the absolute risk from any single chemical exposure remains small. But because these chemicals are ubiquitous and the developing fetal reproductive system is exquisitely sensitive to hormonal disruption, this area of research has drawn serious attention from public health agencies.
Vanishing Testes
In a small number of boys, a testis that was presumably present at some point during fetal development is found to be absent or reduced to a tiny nub of scar tissue. This is called testicular regression syndrome, or vanishing testis. The prevailing theory is that the testis formed normally but lost its blood supply at some point before or just after birth, leading it to wither away. Pathology of these remnants typically shows fibrous tissue with signs of old bleeding, such as iron-containing immune cells and calcification.31PubMed Central. Vanishing testes: a literature review
The management question is whether the remnant tissue needs to be removed. Because a vanishing testis theoretically could harbor surviving germ cells with malignant potential, surgical removal has traditionally been the standard approach.32Scientific Reports. Management of pediatric vanishing testes syndrome based on pathological diagnosis: a single-center retrospective study However, large pathology reviews have found surviving germ cells in only a small fraction of cases, leading some researchers to argue that removal may not be necessary when the remnant is in the scrotum or inguinal area rather than deep inside the abdomen.33Frontiers in Pediatrics. Histopathological Features of Vanishing Testes in 332 Boys: What Is Its Significance? A Retrospective Study From a Tertiary Hospital The debate continues, but either way, boys with a vanishing testis can live healthy lives; the remaining testis generally compensates in terms of hormone production.
The Psychological Side
Most conversations about undescended testes focus on cancer and fertility, but there is a psychological dimension too. Even in adulthood, the absence of a testicle can affect how someone feels about their body. A population-based study of testicular cancer survivors who had a testicle removed found that about a third reported long-lasting feelings of loss, and about a quarter experienced ongoing uneasiness or shame about their body. Men who had never been offered a prosthetic testicle were roughly twice as likely to report these feelings compared to men who had been offered one but chose not to get it.34PubMed. Feelings of loss and uneasiness or shame after removal of a testicle by orchidectomy: a population-based long-term follow-up of testicular cancer survivors That finding is noteworthy: simply being given the option of a prosthesis seemed to matter, even when the man did not take it. For boys who lose a testis to vanishing testis syndrome or surgical removal, or who end up with asymmetric testes after treatment, the option of a prosthetic implant is something worth discussing with a care team, ideally before the issue becomes a source of distress rather than after.