I know, this puts your ambitious career on hold for a while and start ups require huge ambition. Still, start up opportunities will not be hyper critically different in the future than they are now. In the future you may not be able to have children.
This is so completely clear based upon my commute to work. Closer to the downtown where I live there are all kinds of clinics for young mothers in mostly poorer areas. Where I work is the wealthiest county, per capita, in Texas and there are fertility clinics lining the street. That difference is striking. People who put their careers first tend to have more trouble having children and are willing to pay massive sums of money to fix biology.