The OED is unhelpful but wikipedia offers:
‘ . . Of children as of procreation— the pleasure momentary, the posture ridiculous, the expense damnable . . ’
Letter to Nancy Mitford, May 5, 1954, cited from Mark Amory (ed.) The Letters of Evelyn Waugh (Harmondsworth: Penguin, 1982) p. 423
"The pleasure is momentary, the position ridiculous, and the expense damnable" is sometimes attributed to Lord Chesterfield (British statesman, diplomat and wit, 1694-1773), but has not been found in his works.
See also: www.quote-unquote.org.uk/p0000046.htm