Dyfed-Powys Police took to social media on Tuesday to ask for help in tracing Rebecca, saying officers were "very concerned for her".
The force said the mother and child were found in the early hours of Wednesday.
A spokesman said it was thanks to the public response on Facebook.
"Our communities' response to this appeal was significant, with over 1,000 Facebook shares achieved in the first hour and with all parts of Pembrokeshire covered according to the comments posted by our concerned followers," he added.
A missing woman and her baby in Pembrokeshire have been found safe and well after a police Facebook appeal was widely shared.