The old relationship between class and birth rates has been inverted:

Increased prosperity used to lead to a decline in the fertility rate as parents did not need children as an insurance policy for their old age; and indeed, the modern child is very expensive to bring up. But now better-off people seem to be having more children; in the U.S., the fertility rate of wives whose husbands are in the top decile of income is back where it was a century ago. Having a lot of children may be a sign of status – [BCA Research] dubs this the “Brangelina effect” – or it may be that better-off women can afford the childcare help (and increased housing space) that children necessitate.

Interestingly, the proportion of childless highly-educated American women (those with Ph.Ds) aged 40-44 was just 23 percent in 2006-08, down from 34 percent in 1992-94. There was a similar (if less marked) fall in childlessness among those with a master’s degree. … [But] while better-off women may be having more children, that’s not true of the poor; 15 percent of those failed to graduate high school are childless in their 40s, compared with 9 percent 20 years ago and even high school graduates have seen a rise in childlessness from 13 percent to 17 percent. There are a lot more women in those categories than there are Ph.Ds.