The Population Council hired him in 1956 as assistant medical director for its biomedical research laboratories and he was named as director in 1963. Following a two-year residency in India in the 1960s in which he helped establish a model for women's clinics for family planning that would offer contraceptives and prenatal care and set up the first Department of Reproductive Physiology at the All-India Institute of Medical Sciences in New Delhi, Segal came to the conclusion that his career goal should be "to develop practical advances for women's health". He established the International Committee for Contraception Research in 1970 to help provide information regarding the development new contraceptives worldwide. He joined the Rockefeller Foundation in 1978 as the organization's first director of the division of population sciences. He was rehired as Distinguished Scientist by the Population Council in 1991, where he chaired the Institutional Review Board and advised program directors. Segal developed intrauterine devices based on copper and vaginal rings. His best-known work was the creation of Norplant, introduced in 1991 as a contraceptive device that could be inserted under the skin, with its silicone rods releasing progestin for as long as five years. While it offered the benefit of never having to remember to take a pill, side effects included bleeding, hair loss and weight gain. After cases occurred where judges ordered the implant of the device to prevent pregnancy and in the wake of editorials advocating its use "as a way of reducing the welfare burden resulting from high fertility among the underclass", Segal wrote a strongly worded letter to The New York Times in January 1991, indicating that he was "totally and unalterably opposed to the use of Norplant for any coercive or involuntary purpose" and would lead the opposition to use "Norplant for coercive sterilization or birth control". Wyeth-Ayerst Laboratories withdrew the product in 2002 after court cases that argued that a number of major side effects had not been properly disclosed. In 2003, the intrauterine deviceMirena, developed under his leadership of the Center for Biomedical Research, which uses a progestin to thicken cervical mucus to prevent pregnancy was introduced by Berlex Laboratories. Over the years, he helped convince the Chinese government to use more flexible copper-based IUDs, which led to a reduction in the number of abortions necessitated by the use of one-size-fits all steel IUDs. He was the author of more than 300 works, including the 1999 book Is Menstruation Obsolete? which he co-wrote with Elsimar M. Coutinho, which argued that there is no health benefit from monthly menstruation. The book stated that early women would menstruate very infrequently because of frequent pregnancies, child nursing and physical demands and that the side effects of the "incessant ovulation" that modern women experience could include anemia and endometriosis. Segal was awarded the UN Population Award in 1984, sharing it with Carmen Mirro of Panama.. In 2006 he won the Prix Galien USA Pro Bono Humanum Prize along with the Population Council for his inventorship of Norplant.
