Something's wrong here.  Where are the children born during the first decade of their marriage?  They're not in the 1850 Census with her.  Are they away at school?  Did she lose them all?

1.  LDS.  Family Search: Internet Genealogy Service:  AF - Ancestral File (online at FamilySearch.org).

Isaac Dunn BRUCE Born: 30 Oct 1813; Place: Knox, Indiana Death: Aft 1840; Place: Cedar, Iowa Married: Aug 1835 Father: William BRUCE; Mother: Sara "Sally" POLK Spouse: Hetty TRIMBLE

2.  Census Index:  U.S. Selected Counties, 1840 (Broderbund CD-316):

1840

Bruce, Isaac D.

IA

Cedar Co.

4200001

I haven't a clue what these numbers mean; they are not in the standard format for the Federal Census.  Was this a state census?

3.  Census Mircrofilm Records:  Indiana, 1850 (Broderbund CD-302, Disk 3): District No. 61, Knox Co., IN, p. 169, 200/200, enumerated 29 Jul 1850 (Diana Gale Matthiesen).  This census taker's handwriting is very difficult to read — and, yes, he did use "Ia" as the abbreviation for Indiana:

James Foreman	43	M	Farmer	1200	Ia
Rachael	36	F			Ky
Sarah I?	18	F			Ia
Catharine S?	16	F			Ia
Elleanor	10	F			Ia
Nelly Bruce	38	F			Ia
Margaret A	5	F			Ia
Mary	2	F			Ia
James B	6/12	M			Ia

Apparently, Nellie has been recently widowed/divorced/deserted and is living with her sister, Rachel (TRIMBLE) FOREMAN.  It is hard to reconcile Rachel being born in KY and Nellie in IN.    The older sister has to be the one born in KY and the younger one in IN because that is the family's migration route (or could they have gone back and forth?).  Rachel could be lying about her age, or the census taker could have made a mistake.  Given the sloppiness of the handwriting, it's easy to believe this census taker was careless.  On the other hand, given James FOREMAN's age, my guess is that Rachel is lying about hers.  I'm just surprised Nelly isn't.  Of course, Rachel may have been the one being interviewed.  <g>