Initially babies lose weight. This is mainly because they are expelling the black tarry stool (meconium) that has built up inside the colon during pregnancy. After the meconium is flushed out, the baby's weight will stabilize and the baby begins to gain weight. This most often happens after the third or fourth day. Many babies lose about 7-8% before they start gaining. A baby is expected to return to his birth weight by 10-14 days of age.