Perhaps it is Ann, then as these are extracted christenings for an Isaac father St Giles Cripplegate, London:
Isaac FIELD July 29 1787 with mother as Ann
Henry FIELD March 15 1789 - Ann
Rosetta FIELD May 22 1791 - Ann
John FIELD - April 19 1795 also says mother Nelly same day as Thomas
I wonder if you might be related to the famous/infamous John Field, curator of the church in the 16th century, Also there are only a small number of Field births in that church in the 19th century, so if this turns out to be your man, many could be family...None with the name Isaac, however
Guess we may need to look for the spelling as well as found this one
Isaacke FEILD b. May 7 1664 ( christened May 15 / Death Sept. 4 )
St Giles Cripplegate, London / parents Robert & Ann
also noted that 16th century spellings range from Fielde, Feeld, Feelld, feeild, feeild, to Fyelde, Fyelde, Fylde, Flyde, feld, feild, feilde,
Just realized, this is a very old query, so you probably have this