Your baby continues to put on weight and his organs will finish maturing during the third trimester. Here's a detailed look at his development in the final three months: Weeks 26 to 32. During the first six weeks of the third trimester, your baby will more than double in sizeâgrowing from approximately 1.5 to 3.5 pounds. He will also grow in length from about 14 inches to almost 17 inches. The baby's growth during this period is an important indicator of his health.