The answer to this question is that we don't know for sure, because there aren't enough records to tell exactly what the population average was.
However, what we *can* tell you, from a primary source of the extensive diaries of Martha Ballard, a midwife in rural Maine between 1785 and 1812, among her patients 38% of them had conceived before getting married.
Among Ballard's duties as a midwife was to testify in paternity suits. A 1668 law required midwives to ask unmarried women in labour the identity of the father of the baby, with the rationale that nobody would lie while giving birth.
Once she had "taken testimony", Ballard would give evidence in court, and usually the parents of the baby would marry.
OLD THREAD REPOST In 1758, Benjamin Franklin published an instructional manual containing matters "more immediately useful to us Americans." The book contained advice on all sorts of topics including writing, bookkeeping, arithmetic... and a recipe for inducing abortion.
One of these cases involved Ballard's own son, Jonathan. Ballard attended a woman called Sally Pierce in labour, and Sally named Jonathan as the father of the baby. The couple later married, and had a grand total of 13 children together.