The neck gets longer and pulls the head further up away from the chest. The skeletal system is getting harder as your baby takes in nutrients from Mom, namely calcium. If you did a x-ray or even looked through the baby's very thin skin, you would see the outline of a skeleton. Fine hairs called lanugo also cover the body. The baby continues to grow and take up more space in the uterus while showing a decided flair for movement.
Less space means an elbow here, a foot there.
You, therefore, may experience movements for the first time, called fluttering. During week 17 your baby starts to form fat. This is vital for the baby's heat production and metabolism. The rapid growth rate of previous weeks slows down. Finite features of the heart including ventricles and chambers should be visible during an utltrasound and some abnormalities can be detected.