Matthias Jacob Schleiden helped develop the cell theory in Germany during the nineteenth century. Schleiden studied cells as the common element among all plants and animals. Schleiden contributed to the field of embryology through his introduction of the Zeiss microscope lens and via his work with cells and cell theory as an organizing principle of biology.

Leonard Colebrook was a physician who researched bacteria and infections in England during the twentieth century. In 1936, Colebrook deployed the antibiotic Prontosil to treat puerperal fever, a disorder that results from bacterial infections in the uterine tracts of women after childbirth or abortions. Colebrook also advanced care for burn patients by advocating for the creation of burn units in hospitals and by using antisepsis medication for burn wound infections. Colebrook’s work on treatments for puerperal fever reduced cases of puerperal fever throughout the world.

Katharina Dorothea Dalton was a physician in England in the twentieth century who defined premenstrual syndrome (PMS) as a cluster of symptoms suspected to begin one to two weeks before menstruation and disappear upon the onset of a new menstrual cycle. Prior to Dalton, there was little research on pre-menstrual issues and those that existed linked the problem to excessive water retention or estrogen. Dalton hypothesized that PMS resulted from a deficiency in the hormone progesterone and advocated for hormone replacement therapy to lessen the symptoms of the syndrome. Dalton established an early PMS clinic in London, England, and she testified on behalf of women in over fifty court cases claiming to have committed crimes while suffering from PMS.

Anthony Comstock was a US postal inspector and politician who advocated for the suppression of obscenity and vice throughout the late nineteenth century and early twentieth century. Comstock considered any sexually explicit material like pornography and literature related to birth control and abortion as obscene. In 1873, Comstock lobbied US Congress to pass an anti-obscenity law titled An Act for the Suppression of Trade in, and Circulation of, Obscene Literature and Articles of Immoral Use, also called the Comstock Act. The law penalized individuals for sending material classified as obscene through the US postal services, which Comstock, as a special agent of the United States Post Office, could enforce. Comstock’s role in passing and enforcing the Comstock Act influenced the social and political restriction of birth control, hindering women’s access to contraceptives.

Josef Warkany studied the environmental causes of birth defects in the United States in the twentieth century. Warkany was one of the first researchers to show that factors in the environment could cause birth defects, and he helped to develop guidelines for the field of teratology, the study of birth defects. Prior to Warkany’s work, scientists struggled to explain if or how environmental agents could cause birth defects. Warkany demonstrated that a deficiency or excess of vitamin A in maternal nutrition could cause birth defects. He also established that mercury in teething powders increased infant mortality rates. Warkany showed how substances outside the human body could adversely affect conception, growth, and development of the human fetus in utero.

Bernard Sachs studied nervous system disorders in children in the United States during the nineteenth and twentieth centuries. In the late 1880s, Sachs described the fatal genetic neurological disorder called amaurotic family idiocy, later renamed Tay-Sachs disease. The disorder degrades motor skills as well as mental abilities in affected individuals. The expected lifespan of a child with Tay-Sachs is three to five years. In addition to working on Tay-Sachs disease, Sachs described other childhood neurological and developmental disorders that stem from problems with early brain development. Sachs's research in early brain development helped establish some of the physiological symptoms of hereditary neurological disorders in children, enabling doctors to diagnose the disorders earlier and to develop treatments for them.

Henry Morgentaler was a physician who performed abortions, acted as a reproductive rights activist, and advocated for legal access to abortions in Canada during the twentieth century. In 1969, he opened his first abortion clinic in Canada and participated in the legal/court case of R v. Morgentaler (1988), which led Canada to decriminalize abortion. Morgentaler helped establish legal access to abortions for women in Canada and advocated for the protection of women's reproductive choices under the law.

John George Children described several species of insects and animals while working at the British Museum in London, England, in the eighteenth and nineteenth centuries. Children also conducted research on chemical batteries called voltaic cells and briefly studied and manufactured gunpowder. One of the species he described, the Children’s python, or Antaresia children, was used in the twenty-first century as the subject of experiments that involved the biological cost of reproduction in snakes. Those experiments helped examine the importance of thermoregulation during gestation as a possible reason for the evolution of live birth in previously egg-laying species. By researching the Children’s python, Children contributed to the many species of animals used to research reproductive physiology.

In the late nineteenth century, August Karl Gustav Bier was a surgeon in Germany who studied spinal cord anesthesia, later called spinal block. Bier found that, depending upon the amount of anesthesia introduced into the spinal cord, a large area of the human body could be numbed to various degrees. Bier established a procedure to numb individuals from the lower legs to the upper abdomen, with the individual’s numbness ranging from them feeling pressure on their body to them feeling nothing at all. Bier’s spinal block was one of the first instances of regional anesthesia, or numbing a large area of the body, as opposed to only local anesthesia, or numbing a small area of the body. Bier’s development of spinal anesthesia led to improved pain management during major surgeries, including cesarean sections and amputations.

Richard Doll was an epidemiologist and public figure in the twentieth and early twenty-first centuries. Working primarily at the University of Oxford, in Oxford, England, Doll established a definitive correlation between cigarette smoking and lung cancer. Furthermore, Doll’s work helped legitimize epidemiology as a scientific discipline. Doll’s research also helped establish modern guidelines for oncological studies, as well as for contemporary and future research on the effect of smoking on pregnancy and fetal development. In addition to studying the health effects of smoking, Doll also studied cervical cancer and contraceptives.