I did a story based on the murder of my grandmother's younger brother, but it was 1912, a far easier time to get info than the 1700s. However, the execution of women is historically so rare that local records, historical society, even museums, may have details. The woman's exact birth date is essentially irrelevant unless for some reason you want to make reference to her age when executed. But for a "based on" story you need not be accurate. If the murdered child was, say 6, and she is executed 2 years later, don't pick an unlikely age at execution of 15 (gave birth at the age of 7??!), nor of 65.