28 weeks

From PregnancyWeekly.com:
"Your baby weighs approximately 2.4 pounds and is about 10 inches from crown to rump, or a little under 16 inches from head to toe. He or she has been gaining weight steadily during the past 27 weeks as its stem cells develop into organs, blood and immune systems, and bones. However, from this point forward, your baby's weight gain will be due to increasing amounts of body fat. A quick way to predict your baby's birth weight is to average your partner’s and your birth weights."