Snoozing, stretching, swallowing, and even thumb-sucking, your fetus is busy this month as he tests out his new reflexes and abilities. He is losing his top-heavy look as his height starts to catch up to his head size. By the end of this month, he will measure about six to eight inches in length and weigh approximately 6 ounces.

You and your fetus in the fourth month of pregnancy.

Now is a good time to begin singing, reading, and even playing music for your little one. The inner ear structures that allow him to hear are developing this month. He has grown eyebrows, eyelashes, and possibly even a little hair up on top.

The long bones of his arms and legs are growing, as cartilage is replaced with spongy, woven, soft bone in a process called ossification. Skeletal development will continue long after birth and well into adolescence and young adulthood.

Your baby is inhaling and exhaling amniotic fluid, practicing his technique for his first breath in the outside world. The lungs are already generating cellular fluid and a substance known as surfactant. In later months the surfactant will assist the development of the fetal lungs by expanding the alveoli (or air sacs) within them. These substances move out through the trachea and become part of the amniotic fluid, along with the urine your unborn child is already passing.

The umbilical cord contains two arteries and a vein sheathed with a gelatinous tissue known as Wharton's jelly. The jelly cushions the blood vessels and protects them from kinks and twists. Although umbilical cord knots do occur, they are relatively rare, happening in approximately 1 percent of pregnancies.

The placenta is approximately three inches in diameter this month; the attached umbilical cord is about as long as the fetus and continues to grow. Fetal blood is being pumped through this little body at about four miles an hour, exiting through two large arteries in the umbilical cord and on to the placenta. In the placenta, baby's waste products (urine and carbon dioxide) are exchanged for oxygenated, nutrient-rich blood that is returned to the fetus via an umbilical cord vein. Pressure from the blood pumping within the cord helps straighten it out and keeps it from becoming knotted or getting in the way of your unborn baby's kicks and somersaults. Total time for this complex exchange? About thirty seconds.