When did teen pregnancy started to occur the most?

Answer 1

Teen pregnancy was very common when women were often married at 15 and 16 years of age, as was the case before about 1900 in western countries. A girl would be married very young, and would have had two or three children by her twentieth birthday.

There was then a period of increasing wealth and conservatism, when women married a bit older and had children in their twenties, until about 1965, when the rise of hippie culture, and "free love" started up. Since then teenage pregancy outside of marriage has been common.