Teenage pregnancy is defined as a teenaged or underaged girl (usually within the ages of 13–19) becoming pregnant. The term in everyday speech usually refers to women who have not reached legal adulthood, which varies across the world, who become pregnant.

The average age of menarche (first menstrual period) in the United States is 12 years old, though this figure varies by ethnicity and weight, and first ovulation occurs only irregularly until after this. The average age of menarche has been declining and continues to do so. Whether fertility leads to early pregnancy depends on a number of factors, both societal and personal. Worldwide, rates of teenage pregnancy range from 143 per 1000 in some sub-Saharan African countries to 2.9 per 1000 in South Korea.

Pregnant teenagers face many of the same obstetrics issues as women in their 20s and 30s. However, there are additional medical concerns for younger mothers, particularly those under fifteen and those living in developing countries. For mothers between 15 and 19, age in itself is not a risk factor, but additional risks may be associated with socioeconomic factors.

Data supporting teenage pregnancy as a social issue in developed countries include lower educational levels, higher rates of poverty, and other poorer "life outcomes" in children of teenage mothers. Teenage pregnancy in developed countries is usually outside of marriage, and carries a social stigma in many communities and cultures. For these reasons, there have been many studies and campaigns which attempt to uncover the causes and limit the numbers of teenage pregnancies. In other countries and cultures, particularly in the developing world, teenage pregnancy is usually within marriage and does not involve a social stigma. Among OECD developed countries, the United States and New Zealand have the highest level of teenage pregnancy, while Japan and South Korea have the lowest.