In the 1800s women were expected to get married early in life, with many of them being married before the age of 20 years old. This was necessary because the life expectancy was significantly smaller back then that it is now. The average household consisted of seven children and as the years went by this number declined.
In the 1800s and the early 1900s, women only job was to be a good mother and a good wife.
As women started to join the workforce and the cost of living rose to the point where both parents needed to work to raise a family firmly planted in the middle class, teen pregnancy rates started to decline.
College would soon replace marriage and children, and teens no longer married before becoming pregnant so there was not financial stability or society-approved definition of family.