Who's the they here? If you meant than the women wanted, that's only true for some women. Part of the decline is that fewer women who never wanted children are being pressured into having them in the first place and for others, improvements in reproductive education and birth control means they're having the exact number they wanted rather than dealing with accidents, and for still others they wanted as many as they could easily afford (time and money) all along rather than a specific number so they're still getting exactly what they wanted all along too.