Indexes such as Ancestry, Findmypast & Familysearch are incomplete, and sometimes inaccurate, while sometimes no relevant records survive (or perhaps ever existed). One need to review all the evidence and perhaps the result will be no more than an intelligent guess.

So first a review of what we think we know about Sarah Hedges's origins. She was born about 1790 in London - by the mid-19th century this would have included many of the parishes in the south of Middlesex which had been included in the urban sprawl, but excluded the parishes nearest Hertfordshire. For some reason she came to St Albans and had some illegitimate children in the parish of St Michael's before marrying in 1822 in the parish of St Michael's. She, and some of the children spent time in the old St Michael's Workhouse, prior to the formation of the St Albans Union Workhouse. She was almost certainly unable to read and write and may have been a bit vague about her age.

The problem is that surviving records about poor unmarried females are extremely limited and the possible sources of information, in the absence of an obvious baptism, are limited. The fact that any records mentioning inmate names for the old St Michael's Workhouse survive is unusual, and it is even more unusual for details of such surviving records to be available on the internet (Children of St Michael's Workhouse, St Albans). The chances that one of Sarah's children was selected for a case study is about as unlikely as a big lottery win - so you already know very much more than most people will ever know about their "workhouse" ancestors.

So where may more information be found.

The marriage register entry in 1822 will provide the parish of the couple (usually the bride is married in her own parish), their status if married before, their signatures or marks (could they write their own names) and witnesses who may be relatives. You said you were getting a copy - but I am not sure whether you were buying a copy from HALS or viewing the microfilm at your nearest LDS Family History Centre (address on familysearch web site).. Perhaps you are now in a position to let me know what it says.

The baptism registers may provide little more than the familysearch index, but if they were born in the workhouse one would learn nothing new. The best clue are that children are often called after the family - so a daughter may be named after a grandmother - although this is no more a likely suggestion. A son may be named after the otherwise unidentified father - or a male member of the mother's family.

For an illegitimate pauper birth this would have been a matter for the Overseers of the Poor prior to the formation of the Workhouse Unions. In some case they would want to recover expenses from the mother's birth parish, or the father if known. Sometimes a simple entry in a cash book can provide a clue to identity - for one of my own ancestors the only documentary evidence naming the father is the payment he made to the Overseers. The article Children of St Michael's Workhouse, St Albans makes it clear that the admission records for the St Michael's Workhouse have not survived but the reference to the filiation order for William Hedges shown that some other possible records may have survived, and are held in St Michael's parish church. Your best bet would be to contact the author of the article through the History Pieces web site.

Another indirect source is to look for other members of the family. living in the area. For instance Sarah might have come to St Albans as a child when her parents moved to the area. One trick for people born after about 1780 is to look at the census for other people with the relevant surname who were born in the same area who might be parents and siblings. Unfortunately a search for Hedges born before 1800 in London who were living in St Albans in 1851 drew a blank. A similar search of the Burial Index for 1800-1850 might come up with a few names. Of course this does not prove parentage - but once you have target names this may suggest other sources.

Sorry I can't be more helpful but the lack of anything like complete birth/baptism records before 1837 can prove difficult and unfortunately many family trees end at this point. Further research with original documents (some not yet even microfilmed, much less online) at HALS might turn up something, as may the documents held in St Michael's church.  You may get some ideas from the book Tracing Your Family History in Hertfordshire.