From the article, which can be read in its entirety here:
"Mothers of three children stress more than moms of one or two, while mothers of four or more children actually report lower stress levels, according to an exclusive TODAYMoms.com survey of more than 7,000 U.S. mothers released Monday. Call it the Duggar effect: Once you get a certain critical mass of kids, life seems to get a bit easier."
And, just in case you were thinking that four kids would actually be easier than three...