We're never going to know. "shotgun weddings" happened all the time, and still happen today. They're not always obviously "shotgun weddings" and usually aren't advertised (for obvious reasons). Within my own family history, we have many "shotgun weddings", but only discovered by doing some math between birth dates and wedding dates.

I'm not sure anybody can speak to how many marriages were forced by the father of the bride, but there were some weddings that took place when the bride was pregnant. Thomas Purvis (Colonial America To 1763) says, however, that there were not many - about 6% of children born through 1680 were probably conceived before the wedding (compared to 21% in England). In the 18th century, these rates rose to 25%-30% on average in New England and 35% in England.