A 10 year old can become pregnant through sexual intercourse with an adult. The average age of first menstruation is 12, but some girls start as early as 8 or 9. If a girl has sexual intercourse before she starts menstruating, she can become pregnant. In rare cases, girls may start ovulating before they get their first period. This is called precocious puberty and can lead to pregnancy if the girl has unprotected sex.