One story of Sarah: Sarah was the daughter of William's first wife, who died before 1620. After her death, Sarah eloped with Henry Rowley when she was only 14, incurring the wrath of her father. William and Frances may have been planning to accompany her father on the Speedwell in 1620, but instead delayed their departure. William was still upset over 10 years later when he made his will and left out Henry (Sarah was dead) and their children except for Moses.
Another theory: As Sarah Palmer was expecting when she and husband Henry Rowley came to America, they brought along a young nanny named Mary Trine. (This later became a wife of Robert Paddock.) After Henry Rowley married Ann (Elsdon/Helsdon/Heilsdon) Blossom, Mary Trine moved into the William Palmer household as a maid servant and got into trouble with a man servant of William Palmer, Sr. in 1633.
It does make sense that Sarah was expecting in the summer of 1632 -- Moses Rowley was an heir in his grandfather Palmer's will. He was granted a cow out of the estate on March 7, 1653/54, an occasion which may correspond with his 21st birthday.
From Rowley Registry: Henry Rowley, Planter of Plymouth, had at least two wives. It has been popularly recorded that his first wife was Sarah Palmer. Documentation that her name was Sarah has not been forthcoming, although it is almost certain that she was a daughter of William Palmer, Nayler of London and colonist of Plymouth. It is not known precisely when she died. It has been alluded to that she succumbed to the same "infectious disease" that felled Thomas Blossom.