Few arguments about pain are as stubborn or as awkwardly personal as the contest between childbirth and a kick in the testicles. Both can be brutal, but the comparison quickly runs into anatomy, duration, danger and the deeply subjective nature of suffering.

A forceful hit to the testicles can double someone over, trigger nausea or vomiting, and even lead to a hospital visit. The pain may also travel upward into the abdomen rather than staying neatly confined below the belt.

That spreading pain reflects how the testicles develop in the abdomen before descending into the scrotum shortly before or after birth, carrying nerves and tissues along. Their role in sex and reproduction also leaves them packed with nerve endings and acutely sensitive.

From an evolutionary perspective, intense pain encourages people to protect organs that are essential for producing sperm. Unfortunately for their owners, those organs occupy a thin sac outside the body, leaving them unusually exposed to impact and injury.

A 2009 paper in Evolutionary Psychology noted that ovaries outside the body would face damage, insult and temperature variation. The authors said the same reasoning applies to testicles, yet sperm must be kept cool, so the organs remain outside.

Other animals have taken a different anatomical route. Elephants keep their testicles inside their bodies near their kidneys, while humans have external testicles and an upright posture that makes their location particularly precarious.

After a less serious blow, the pain may subside in about an hour. A sufficiently hard impact, however, can cause an injury serious enough to require hospital treatment, making the familiar instinct to cross one’s legs entirely understandable.

Childbirth presents a very different ordeal. It pushes a human through a passage with a baseline width slightly greater than 3 centimeters, or 1.2 inches, and historically killed roughly one in 25 women who gave birth.

For humans, a first birth takes an average of nine hours, about 30 times as long as birth in a horse. Human labor is also notably painful and complex compared with birth among the great apes, and humans require assistance during delivery.

Bioanthropologist Holly Dunsworth said humans have the longest pregnancies among primates when maternal size is considered, along with the biggest babies and brains. Those physical demands help make human pregnancy and birth an exceptional burden on the mother.

One explanation is known as the obstetric dilemma, which describes competing evolutionary pressures involving large brains and an upright posture. Scientists disagree about the relative importance of those pressures, particularly because human hips could be wider without necessarily preventing efficient walking.

During labor, uterine muscles contract involuntarily while the cervix and vagina stretch to permit delivery. Bones may be forced aside as the baby moves, and the person giving birth can suffer tearing that extends from the vagina toward the anus.

“A labor contraction is just one big muscle cramp, as the entire uterus contracts,” Bart Putterman, an OB-GYN at Texas Children's Pavilion for Women in Houston, told Parents. The pain may be felt anywhere the muscle contracts, while the baby’s position can contribute to back pain and cramping.

Duration adds another punishing dimension. Unlike a sudden kick, labor can continue for hours or even days, grow progressively more painful, and exhaust both body and mind after nine months of anticipation.

Nearly two thirds of women in the United States report a pathological fear of giving birth, a condition called tokophobia. That fear exists alongside a reported maternal death rate of about 33 per 100,000, and the source notes that the figure is not improving.

Pain can continue after delivery through soreness, stitches and continuing contractions. Those contractions help limit excessive blood loss and return the uterus from a volume that increased 500 times to its regular size.

Ultimately, pain cannot be ranked with a universal measuring stick because individual experiences vary. Childbirth lasts far longer and carries extensive physical demands, while a testicular blow delivers concentrated agony to an exposed organ, leaving the contest at an unsatisfying tie.