Do not choose your medical school based on what you perceive *might* be your field of choice years from now. Reasons being:
1) many of those who begin medical school wanting to do "Specialty X" end up doing something else
2) as others have noted, all US medical schools, and most of those abroad will provide you with adequate exposure to OB-Gyn during your Core rotations.
3) despite any interest you may have in the field, you will not gain anymore exposure or be anymore competitive coming from School X vs School Y.
4) with the exception of a notable *very* few schools, the name brand does not significantly alter your application and ability to match and match well into the field of your choice. Your application is evaluated on your USMLE scores, LORs, clerkship grades and evals.
Choose the medical school at which you would be happiest (ie, the cheapest one, the one closest to home, the one in a city you've always wanted to live in, the one with the friendliest students or one with a particular faculty member you've always wanted to work with), not for some perceived benefit in matching which may not be real.