I wonder if this is a match for Stephen the elder's wife Sarah Thomas; her gravestone lists a birthday of 10 May 1816; this baptism record is 4 days later:
Sarah Thomas Wales, Carmarthenshire, Parish Registers Name: Sarah Thomas Event Type: Baptism Event Date: 14 May 1816 Event Place: Llanegwad, Carmarthenshire, Wales Digital Folder Number: 004213946
no subject
Sarah Thomas
Wales, Carmarthenshire, Parish Registers
Name: Sarah Thomas
Event Type: Baptism
Event Date: 14 May 1816
Event Place: Llanegwad, Carmarthenshire, Wales
Digital Folder Number: 004213946