The identity of Martha Leveret is reasonably certain.
Only one suitable baptism was found in FREEREG to Charles and Martha.
It is likely that these were here parents, but by no means certain.
  

Martha was married in 1738.  There are three possible baptisms:
1702		in Morston			to Samuel and Modesty Jarvis
21 Aug 1714	in Barney			to Henry and Jane Jarvis
Apr 1716	in Gunthorpe		to Richard and Martha Jarvis
All three places are near Wighton etc.

Her age when married would be 36, 24 and 22 respectively.
The latter two are more likely but beyond this there is insufficient information.
Searched for evidence of death in infancy of these three, but not found.