Teenage pregnancy, also known as adolescent pregnancy, is pregnancy in a female adolescent or young adult under the age of 20. This includes those who are legally considered adults in their country. The WHO defines adolescence as the period between the ages of 10 and 19 years. Pregnancy can occur with sexual intercourse after the start of ovulation, which can be before the first menstrual period (menarche) but usually occurs after the onset of periods. In healthy, well-nourished girls, the first period usually takes place around the age of 12.

Pregnant teenagers face many of the same pregnancy related issues as other women. There are additional concerns for those under the age of 15 as they are less likely to be physically developed to sustain a healthy pregnancy or to give birth. For girls aged 15–19, risks are associated more with socioeconomic factors than with the biological effects of age. Risks of low birth weight, premature labor, anemia, and pre-eclampsia are not connected to biological age by the time a girl is 16, as they are not observed in births to older teens after controlling for other risk factors, such as access to high-quality prenatal care.

Teenage pregnancies are related to social issues, including lower educational levels and poverty. Teenage pregnancy in developed countries is usually outside of marriage and is often associated with a social stigma. Teenage pregnancy in developing countries often occurs within marriage and half are planned. However, in these societies, early pregnancy may combine with malnutrition and poor health care to cause medical problems. When used in combination, educational interventions and access to birth control can reduce unintended teenage pregnancies.

In 2015, about 47 females per 1,000 had children well under the age of 20. Rates are higher in Africa and lower in Asia. In the developing world about 2.5 million females under the age of 16 and 16 million females 15 to 19 years old have children each year. Another 3.9 million have abortions. It is more common in rural than urban areas. Worldwide, complications related to pregnancy are the most common cause of death among females 15 to 19 years old.

In 2021, 13.3 million babies, or about 10 per cent of the total worldwide, were born to mothers under 20 years old.