Of the many disclosures about the Republican vice-presidential candidate, Sarah Palin, since John McCain selected her as his running mate, one of the most intriguing surrounds the birth of her youngest child, Trig.

Even her closest aides did not know she was pregnant until a few weeks before Trig's birth in April.

That, according to yesterday's New York Times, is because she hid her pregnancy. The 44-year-old Governor was photographed by Vogue in January in a giant parka. At work, she began wearing billowing scarves and loose jackets.

She was, it seems, struggling to come to terms with the fact that the baby would be born with Down's syndrome. She also feared a public backlash over the pregnancy. So she kept working, even travelling to Texas a month before her due date, to give a speech.

On the day of that speech, Ms Palin began discharging amniotic fluid. She went ahead and gave the speech anyway, before returning to Alaska – a 10-hour flight.

"Nobody knew a thing," Governor Linda Lingle of Hawaii told The New York Times. "I only found out from my security detail on the way home that she had gone into labour and that she had gone home to Alaska."

Trig Paxson Van Palin was born early next morning, weighing 6lb 2oz. Mrs Palin soon sent an e-mail to relatives and friends, signing it: "Trig's Creator, Your Heavenly Father."

"Many people will express sympathy, but you don't want or need that, because Trig will be a joy," Mrs Palin wrote. Three days later, the Governor returned to work.

Palin family names explained

*Track, 19, named after the lake where his father fishes.

*Bristol, 17, named after Bristol Bay – where her mother wants to create a huge mine.

*Willow, 14, inspired by willow ptarmigan, Alaska's state bird.

*Piper, 7, bears the same name as the family plane.

*Trig, 4 months – Trig is Norse for "brave victory".