Q: Do I send my original birth certificate and marriage certificate to NVC?
A: No. For NVC processing, you normally submit scans or copies of the required civil documents. Keep the originals or certified copies, because you will need to bring them to the immigrant-visa interview.
Source: U.S. Department of State — Submitting Documents to the NVC: https://travel.state.gov/content/travel/en/us-visas/immigrate/submitting-documents-to-the-nvc.html