I measure my girls on the last day of breeding, right behind the last rib. (As the woodhavens site indicates).
I remeasure again, 4 weeks later, then once a week unitil the last week and on the last week i measure daily.
Its fun, and my girls love the added attention. I use the same tape measure every time, and record all the results so that i don't forget anything.
I have had 100% accurcy (sp?), it saves trips to the vet and saves money.