In poor neighbourhoods where men were the minority, women had children earlier, the investigators found.
But in more affluent neighbourhoods where men were similarly outnumbered, women focused on education and career, later becoming the group with the higher birth rate in their 30s.
Study co-author Abby Chipman said that in poorer neighbourhoods the increase in the birth rate was mainly among 15-to-24-year olds, while in more affluent areas it was among those aged 34 to 39.
“If there are more women than men, studies have shown that women have lower expectations of men,” she said.
“We found that poor women are more likely to rush to start their ‘reproductive careers’, while rich women are more likely to delay having children.