Parish registers were started in 1538, but for many parishes do not go that far back. Older registers will often only include the name of the child, the date of the baptism, and the name of the father (unless illegitimate). Later many registers started to give the mother's name as well, and some ministers included information such as occupation, address, and date of birth as well. Sometimes there will added notes - about private baptisms (often because the child was weakly and not expected to survive) or some extra tit-bit. For this reason (and to check for transcription errors) you should always look at the microfilm of the register and not rely on indexes, such as familysearch.
In 1813 pre-printed baptism books were introduced with columns for "When Baptised", "Child's given name", "Parents Names", "Abode", "Quality, trade or profession", "By whom the ceremony was performed". For some parishes the minister will also have recorded the date of birth as well - either in every case or where the child was no longer an infant. In some cases "Abode" may be no more than the parish - but out of parish baptisms should be noted, and some ministers will have recorded actual addresses - another reason for checking the microfilm. "Quality" refers to the gentry - Lord, Sir, Esquire, etc., - who, of course would not demean themselves by working for a living (even if they did).
Care must be taken if two or more children are baptised at the same time.
During 2012 FindMyPast are to digitise the Hertfordshire Parish Registers and Bishops Transcripts held at HALS up to 1910 for baptisms, 1928 for marriages and 1990 for burials.
Where is my ancestor's baptism before 1837?
It may well be that the relevant parish registers have not yet been entered on the index, they could have been entered erroneously (see the postings on the census to see what kind of things can go wrong), or the parish registers may have been lost or damaged.
The problem is particularly acute when one gets back to 1837 and civil registration and the censuses can no longer help, and in many cases I am afraid no birth or baptism records exist for a large number of people before that date.
One rather obvious reason is that there was no legal reason for getting children baptised so many of the poor did not bother. In addition many people were members of non-conformist chapels in which the children may have been accepted into the congregation in a baptism or other service. These details may have been written into some kind of chapel log book, sometimes mixed up with burials, reports of church meetings, etc. These books were kept by the minister, and when he moved on the book often followed him to his new chapel. Unlike the Church of England registers there was no laws relating to these books, except that some had to be handed in when civil registration was introduced in 1837. Some were handed in, and are now available on microfilm, but the vast majority have been irretrievably lost (although a few may be in private collections).
The scale of the problem can be seen by looking at the places listed in Hemel Hempstead in the 1851 ecclesiastical census.
Church of England - St Mary, ancient parish church [registers survive from 1566] and a chapel of ease at Boxmoor, consecrated 1830
Independent - The Box Lane Chapel at Boxmoor was erected in 1714 [records 1791-1837] Another chapel was erected in 1836.
Baptists - in addition to original chapel, which was erected in 1731 [records survive for the years 1785-1834] there was a Salem Chapel at Two Waters erected in 1818, the Particular Baptists erected a chapel in 1825 and there was a Baptist Meeting House at Water End which had been in use about 16-20 years. No records have survived for these.
Society of Friends - meeting house in Bell yard erected before 1800 [Quaker records were held centrally and are excellent if you are lucky enough to have Quaker ancestors.]
Methodists - There were three Wesleyan Methodist chapels erected in 1839, 1846 and 1846 and a Primitive Methodist Chapel erected in 1849
Latter Day Saints - erected 1850
In addition, before chapels were erected there would often be congregations meeting in private houses or barns, which needed to be registered for the purpose. As far as I know no records have survived for these.
For information on the chapels that existed in Hertfordshire see Religion in Hertfordshire and for the surviving parish registers and chapel records see the National Index of Parish Registers, Volume 9, Part 6, Hertfordshire
Last update April 2007