People are having fewer kids today because of changes in society. Both men and women work more and have less time to raise children. Women have a lot more independence, and they want to have careers first rather than just being stay-at-home moms. The availability of contraceptives also make accidental pregnancies less common.
Declining fertility is not to blame for falling birth rates. Declining birth rates are caused by aging population and economic inequality as well as the change in social norms that have reduced the interaction of individuals to texts and e-mails. The decline in fertility may also be due to environmental factors but it is not the cause of falling birth rates.