In 1900, the fertility rate in the region of present-day South Korea was six children per woman, meaning that the average woman born in South Korea in that year could expect to have six children over the course of their reproductive years. This number began to fluctuate in the 1930s, when the Japanese administration (the Korean peninsula had been annexed by Japan in 1910) promoted fertility as part of the war effort, before fertility dropped below 5.2 births per woman in the aftermath of the war. It then increased above 6.3 in the 1950s due to the devastation and mass-displacement caused by the Korean War. As stability returned to the region, South Korea's fertility rate would fall sharply throughout the remainder of the century, as modernization, urbanization, and the implementation of family planning programs would see fertility fall to just over 1.5 children per woman by 1990.

Sex-selective abortion and gender ratios Abortion was illegal in South Korea between 1953 and 2020, although it was permitted in some cases from 1973 onward. Despite this, these laws were rarely enforced, and sex-selective abortion became widespread following advancements in ultrasound technology. In many Asian societies, it was often preferred to have male children as they were viewed as being better long-term providers for their parents and they would carry on the family name. In South Korea in the early 1990s, the practice of sex-selective abortion became so widespread that the gender ratio at birth was 114 males for every 100 females (reportedly as high as 125 in some cities), compared to the historical and natural average of approximately 105 males per 100 females. The government then prohibited doctors from revealing the gender of unborn babies to the parents in 1987, and introduced more severe penalties in 1994, in an attempt to revert this trend. The gender imbalance then reduced in the following decades, and has been at 106 males per 100 females since the 2010s (roughly the natural average). Abortion rights in South Korea were expanded in 2021.

Lowest in the world? Despite government initiatives aimed at increasing fertility, including financial incentives, South Korea's fertility rate has continued to fall in recent years, and today is at around half of replacement level. In 2020, it is estimated that the average woman born in South Korea will have just over one child over the course of their reproductive years. Some critics cite economic factors, such as high education and housing costs, for the reason that young couples are postponing marriage and having families; today, South Korea has the lowest adolescent fertility rate, and the lowest overall fertility rate in the Asia Pacific region. Due to the current trajectory of South Korea's fertility rate, in January 2021, it was announced that the South Korean population experienced a natural decline for the first time in it's history.

In 2024, the birth rate in South Korea stood at 0.75 births per woman. The country has long struggled with a declining birth rate, dropping below one birth per woman in 2018.

In 1900, the region of present-day North Korea had a fertility rate of approximately 6.1 children per woman, meaning a woman born in North Korea in the given year could expect to have just over six children throughout the course of her reproductive years. This rate would remain largely consistent until the 1940s, however, the middle of the century would see dramatic rises and falls in fertility, as Japan's annexation of the Korean peninsula in the Second World War, and the subsequent Korean War in the early-1950s saw large scale socioeconomic turmoil in the peninsula. Because of this, fertility halved to 3.1 between 1940 and 1950, before spiking to 5.1 by the 1960s.

North Korea's fertility rate fluctuated in the decades following these wars, as the country recovered and came to grips with its new structure; while the total fertility rate began upon a more consistent trajectory of decline from the 1980s onwards. In 2020, it is estimated that the average woman born in North Korea can expect to have approximately 1.9 children over the course of their reproductive years.

In 1900, the child mortality rate in the area of present-day North Korea was estimated to be almost five hundred deaths per thousand live births, meaning that approximately half of all children born at this time were not expected to survive past their fifth birthday. This rate would increase to 524 deaths per thousand births in the 1910s, before rapidly falling from the 1910s until the middle of the century (following the Japanese annexation of the peninsula in 1910). There was a slight increase in child mortality rates in the late 1940s, as the Korean peninsula was divided into two states; although it continued upon its rapid decline in the 1950s, with the decrease slowing in the next three decades.

This decline would continue steadily until the 1990s, when the collapse of the Soviet Union and the resulting cut-off of economic aid would cause child mortality to rise for a decade, and this was exacerbated by the flooding, drought, famine and economic mismanagement of the late 1990s. The past two decades, however, have seen child mortality fall once more, and in 2020, it is estimated that for every thousand children born in North Korea, over 98 percent will make it past the age of five.

In 1890, the average person born in the area of present-day North Korea could expect to live to 26 years old, a rate which would see only marginal change until the annexation of the Korean peninsula by the Empire of Japan in 1910. As Japanese large scale industrialization and modernization reforms would begin in the peninsula (building on the Gwangmu reforms of the previous administration), life expectancy would begin to increase rapidly as standards of living rose across the region. As a result, life expectancy would rise by twenty years between 1920 and 1940, however this growth would slow during the 1940s, as Korea lost approximately half a million people (or 2% of its population) during the Second World War. Following the war, the peninsula was split into two separate states, with the north and south each administered by the Soviet Union and the U.S. respectively (although both militaries had largely left the peninsula within a few years). In 1950, as tensions grew between the two governments regarding the legitimate rulers of the peninsula, North Korea invaded the South and set in motion the Korean War. The war would last for three years, and result in the deaths of approximately three million Koreans (with the North suffering heavier losses than the South); this caused the life expectancy in North Korea to plummet from over 49 years to less than 38 years in the first half of the 1950s.

Life expectancy in North Korea would quickly recover in the post-war years, as the Soviet Union would provide significant economic aid to the country. As a result, life expectancy would return to pre-war levels by the end of the decade and resume its steady rise until the collapse of the Soviet Union in 1991. After peaking at 70 years in the early 1990s, it is estimated that life expectancy fell by nearly seven years in the early 1990s as the country would face extreme medical and food shortages following the end of Soviet aid. U.S. estimates put the death toll of these famines at more than half a million people in the 1990s. In the past two decades, life expectancy has slowly increased once more in North Korea, returning to pre-famine levels once more in 2015, and it is now estimated that the life expectancy from birth in North Korea in 2020 is approximately 72 years old.