Although this might be the secret hope of hundreds of young people in the backs of cars or under the bleachers, wishing something doesn't make it so. The fact remains that whenever there is a chance for an egg and sperm to meet, there's a chance of conception. So if a young woman has begun ovulating (as evidenced by monthly menstruation), and a young man is capable of ejaculation, the odds of making a baby are exactly the same -- whether it's the first time or the hundred-and-first time either party is having sex.
And what exactly are those odds?
Most experts place it at between 15 and 25 percent for each ovulatory cycle -- with the higher percentage numbers being more applicable for women in their 20s [source: Moore].