Girls become physically capable of giving birth after they are able to conceive, which is usually around the age of 12, according to WebMD. The first menstruation marks when a girl begins to release eggs and is able to conceive a child.
Girls typically begin to menstruate between the ages of 11 and 14, explains WebMD. Menstruation, also known as a period, is the shedding of the uterine lining, and happens each month if no egg is fertilized. This means it is possible for a girl to conceive and give birth before her first period if she has intercourse in the time between the release of the first egg and her first menstruation.