Your baby's heartbeat may now be audible through an instrument called a doppler. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails, and hair are formed. Teeth and bones become denser. Your baby can even suck his or her thumb, yawn, stretch, and make faces.The nervous system is starting to function. By the end of the fourth month, your baby is about 6 inches long and weighs about 4 ounces.