I wondered the same during my last preg. First was born at 38 weeks and weighed 5 lbs 12 oz. I was so afraid my 2nd would come earlier but she hung out until 39 weeks 1 day. She weighed 7 lbs 8 oz. I met a number of moms who's second came later. And my friend is 39 weeks pregnant with her third and her second came at 37 weeks.
Mama to DD (1/09)
and DD (9/12)