More than half of the babies born in Wales this year will be to unmarried parents. The same is true in the north part of England. Britain as a whole is likely to pass the 50% threshold within a decade if present trends continue.
In the U.S. we worry because more than a third of all births are to unmarried parents. To see that the illegitimacy rate in the Mother Country is much worse, and about to pass a tipping point, is, in a way, comforting.
I think the crucial difference between Britain and U.S. that affects birth rates is that we are a much more religious people. While it is certainly true that religiosity does not grant any family immunity from out-of-wedlock births, it does make marriage before babies more likely. How much more likely? Looking at these figures, about a fifth more likely.