Notes:
confirmed St. John's Ev. Ref. Ch., Frederick Co. 1788 at the age of 17, thus making her birth about 1771
Data source is the 1990 book, "Moomaw-Mumma-Mumaw-Mumaugh Genealogy" compiled by Robert Austin Moomaw (deceased).
One important record is the birth/baptismal record for Anna Margaretha Mumma, b. 28 Jan 1771, bapt. 7 Apr. 1771 in Emanuel's Reformed Church in Hanover. The parents were Jacob Momma and wife, Anna Marg. (John Jacob Mumma, b. 1735). Sponsors were Johannes Etzler and Juliana Schreiber, both single. This shows that the family left Lancaster County prior to 1771 on their migration to Frederick Co., MD.
I attempted to verfify this birth/baptismal record on Ancestry which has the Emannuel Reformed Church records on-line, but she did not appear. This needs further confirmation.