Assuming a girl doesn't have fertility issues, and is 18, what's the chance of pregnancy cumming inside no birth control? over a year?approximate.

Whoa!: 18 is too young to start having little ones. She has to get at least a couple years of after-high-school education, be married, and land a job. With a job comes medical insurance benefits (babies see the doctor 8 times the first year). So, use that birth control. Healthy 18 year-olds get pregnant faster than a hot knife through butter! Without birth control, most will be pregnant within 6 months.