From the Mayflower History Website, http://www.mayflowerhistory.com by Caleb H. Johnson:
Mary Norris was probably about 30 years old when she came to Plymouth on the Mayflower in 1620, estimated from the known age of her husband, and on her known marriage date. She married Isaac Allerton in Leiden, Holland in 1611, one of the first marriages amongst the Pilgrims after they had fled to Holland from England. Her marriage record indicates she was from Newbury, perhaps Newbury, Berkshire, England. She had children Bartholomew, Remember and Mary in Leiden, all of whom came on the Mayflower with her. She and Isaac buried a child, not yet named, at St. Peters, Leiden on 5 February 1620, and she gave birth to a stillborn son ten months later, on board the Mayflower in Plymouth Harbor, on 22 December 1620.
Mary died on 25 February 1621, during the height of the first winter when half the Mayflower passengers and crew died. Her husband Isaac, and her three children Bartholomew, Remember, and Mary, all survived, however. Isaac would later remarry to Fear Brewster, daughter of Plymouth's Elder William Brewster.