Maria Janetta Nelthorpe, was the only daughter of John Nelthorpe, of Little Grimsby Hall, Lincolnshire, England, and his wife, Mary Cracroft. She was christened at Little Grimsby on 16 October 1775, and was probably born that year. Her second name is sometimes spelled Jenetta or Jeanetta.
She married 4 March 1799 at Little Grimsby, as his second wife, Lord William Beauclerk, second son of Aubrey Beauclerk, 5th Duke of St Albans, and Lady Caroline Cavendish. He succeeded his nephew as 8th Duke of St Albans on 19 February 1816. They had fourteen children;; 
Lady Mary Amelia Beauclerk, born May 1800, christened 10 or 18 May 1800 at Redbourne
William Aubrey de Vere Beauclerk, born 24 March 1801, succeeded his father as 9th Duke of St Albans;
Lady Charlotte Beauclerk, born 4 April 1802, christened 18 April 1802, Redbourne, Lincolnshire;
Julia Catharine Beauclerk, born 1803, buried 17 August 1803 Redbourne;
Lady Caroline Janetta Beauclerk, born 28(?) June 1804, christened 20(?) June 1804 Redbourne;
John Nelthorpe Beauclerk, christened 18 December 1805 Redbourne;
Lady Louisa Georgiana Beauclerk, born 28 December 1806, christened 4 January 1807 Redbourne;
Lord Frederick Charles Peter Beauclerk, born 28 June 1808, christened 6 July 1808 Redbourne;
Lady Georgiana, born September 1809
Lady Mary Noel Beauclerk, born 28 December 1810, christened 7 January 1811 Redbourne;
Lord Henry Beauclerk, born 23 June 1812;
Lord Charles Beauclerk, born 10 October 1813, christened 1 November 1813;
Lord Amelius Wentworth Beauclerk, born 16 August 1815;
Lord George Augustus Beauclerk, born 14 December 1818 and christened 28 December 1818 St James', Westminster, Middlesex
Maria Janetta died 17 January 1822 at St James's Square, London. Her husband died also at 21 St James's Square on 17 July 1825, and was buried in Lincolnshire. His will was proven in November 1825 but administration of her estate wasn't settled until August 1827.
↑ 1.01.11.2 Cokayne, George Edward, The Complete Peerage: or a history of the House of Lords and all its members from the earliest times, 2nd ed., vol. 11, p. 290-92, revised & edited by Geoffrey H. White. London: St Catherine Press, 1949. Digitised edition by ABC Publications, 2003.
↑ "England Births and Christenings, 1538-1975," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:NG5M-MYD : 30 December 2014), christening Maria Jenetta Nelthorpe, 16 Oct 1775 dau of John & Mary Nelthorpe; citing LITTLE GRIMSBY,LINCOLN,ENGLAND, reference ; FHL microfilm 432,515, 507,132.
↑ "England Births and Christenings, 1538-1975," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:N68Z-TQ7 : 30 December 2014), various children of William & Maria Janetta Beauclerk; citing Redbourne, Lincoln, England, reference item 2; FHL microfilm 1,450,447.
↑ Lodge, Edmund, The Peerage of the British Empire as at present existing, London: Saunders and Otley, 1832. p. 248. Digitised image Google Books.