Jerry Hall met Lucian Freud at a dinner in 1997 when she was eight months pregnant with her fourth child. He asked if he could paint her portrait, and she started posing the next day, three times/week, until she went into labor.
I'm assuming that more came out of those painting sessions than a single 4x6 giveaway portrait, but since Jerry's the one selling her art collection, that's what Sotheby's gets to work with.
Oct 15, Lot 14: Eight Months Gone, Lucian Freud, est. 300,000-400,000 GBP [sothebys.com]
update: Oh, this painted postcard sold for 602,000 pounds.