Complex sentence: The number of teenage brides marrying for the first time topped one hundred thousand; the proportion of them pregnant on their wedding day had increased from 25% during the 1950s to almost 40%, more than double the proportion of women in their twenties.
Split into simpler sentences: The number of teenage brides marrying for the first time topped one hundred thousand. The proportion of them pregnant on their wedding day had increased from 25% during the 1950s to almost 40%, more than double the proportion to that of women in their twenties.